diff --git a/AltairZ80/altairz80_net.c b/AltairZ80/altairz80_net.c
index 70be5dd3..d43557b0 100644
--- a/AltairZ80/altairz80_net.c
+++ b/AltairZ80/altairz80_net.c
@@ -122,7 +122,7 @@ DEVICE net_dev = {
 static t_stat set_net(UNIT *uptr, int32 value, CONST char *cptr, void *desc) {
     char temp[CBUFSIZE];
     if ((net_unit.flags & UNIT_ATT) && ((net_unit.flags & UNIT_SERVER) != (uint32)value)) {
-        strncpy(temp, net_unit.filename, CBUFSIZE); /* save name for later attach */
+        strncpy(temp, net_unit.filename, CBUFSIZE - 1); /* save name for later attach */
         net_detach(&net_unit);
         net_unit.flags ^= UNIT_SERVER; /* now switch from client to server and vice versa */
         net_attach(uptr, temp);
diff --git a/AltairZ80/altairz80_sio.c b/AltairZ80/altairz80_sio.c
index 0cf8d87f..cb184c65 100644
--- a/AltairZ80/altairz80_sio.c
+++ b/AltairZ80/altairz80_sio.c
@@ -1357,6 +1357,7 @@ static void setClockZSDOS(void) {
     newTime.tm_hour = fromBCD(GetBYTEWrapper(setClockZSDOSAdr + 3));
     newTime.tm_min  = fromBCD(GetBYTEWrapper(setClockZSDOSAdr + 4));
     newTime.tm_sec  = fromBCD(GetBYTEWrapper(setClockZSDOSAdr + 5));
+    newTime.tm_isdst = 0;
     ClockZSDOSDelta = mktime(&newTime) - time(NULL);
 }
 
@@ -1371,6 +1372,7 @@ static time_t mkCPM3Origin(void) {
     date.tm_hour    = 0;
     date.tm_min     = 0;
     date.tm_sec     = 0;
+    date.tm_isdst   = 0;
     return mktime(&date);
 }
 
diff --git a/AltairZ80/m68kcpu.c b/AltairZ80/m68kcpu.c
index 19310c6e..e181a9b3 100755
--- a/AltairZ80/m68kcpu.c
+++ b/AltairZ80/m68kcpu.c
@@ -51,7 +51,7 @@ const char* m68ki_cpu_names[9] =
     "M68000",
     "M68010",
     "Invalid CPU",
-    "M68EC020"
+    "M68EC020",
     "Invalid CPU",
     "Invalid CPU",
     "Invalid CPU",
diff --git a/AltairZ80/s100_disk2.c b/AltairZ80/s100_disk2.c
index c698a36f..6c23f105 100644
--- a/AltairZ80/s100_disk2.c
+++ b/AltairZ80/s100_disk2.c
@@ -455,34 +455,38 @@ static uint8 DISK2_Write(const uint32 Addr, uint8 cData)
                                       " READ_DATA: cyl=%d, track=%d\n", PCX, disk2_info->cyl, pDrive->track);
                             pDrive->track = disk2_info->cyl; /* update track */
                         }
-                        sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET);
-                        for(i=0;i<pDrive->nsectors;i++) {
-                            /* Read sector */
-                            rtn = sim_fread(sdata.raw, 1, (pDrive->sectsize + 3), (pDrive->uptr)->fileref);
-                            if (rtn != (size_t)(pDrive->sectsize + 3)) {
-                                sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
-                                          " READ_DATA: sim_fread error.\n", PCX);
-                            }
-                            if(sdata.u.header[2] == disk2_info->sector) {
-                                if(sdata.u.header[0] != disk2_info->cyl) { /*pDrive->track) { */
-                                    sim_printf("DISK2: " ADDRESS_FORMAT
-                                           " READ_DATA Incorrect header: track" NLP, PCX);
-                                    disk2_info->timeout = 1;
-                                }
-                                if(sdata.u.header[1] != disk2_info->head) {
-                                    sim_printf("DISK2: " ADDRESS_FORMAT
-                                           " READ_DATA Incorrect header: head" NLP, PCX);
-                                    disk2_info->timeout = 1;
+                        if(sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET) == 0) {
+                            for(i=0;i<pDrive->nsectors;i++) {
+                                /* Read sector */
+                                rtn = sim_fread(sdata.raw, 1, (pDrive->sectsize + 3), (pDrive->uptr)->fileref);
+                                if (rtn != (size_t)(pDrive->sectsize + 3)) {
+                                    sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                              " READ_DATA: sim_fread error.\n", PCX);
                                 }
+                                if(sdata.u.header[2] == disk2_info->sector) {
+                                    if(sdata.u.header[0] != disk2_info->cyl) { /*pDrive->track) { */
+                                        sim_printf("DISK2: " ADDRESS_FORMAT
+                                                   " READ_DATA Incorrect header: track" NLP, PCX);
+                                        disk2_info->timeout = 1;
+                                    }
+                                    if(sdata.u.header[1] != disk2_info->head) {
+                                        sim_printf("DISK2: " ADDRESS_FORMAT
+                                                   " READ_DATA Incorrect header: head" NLP, PCX);
+                                        disk2_info->timeout = 1;
+                                    }
 
-                                selchan_dma(sdata.u.data, pDrive->sectsize);
-                                break;
-                            }
-                            if(i == pDrive->nsectors) {
-                                sim_printf("DISK2: " ADDRESS_FORMAT
-                                       " Sector not found" NLP, PCX);
-                                disk2_info->timeout = 1;
+                                    selchan_dma(sdata.u.data, pDrive->sectsize);
+                                    break;
+                                }
+                                if(i == pDrive->nsectors) {
+                                    sim_printf("DISK2: " ADDRESS_FORMAT
+                                               " Sector not found" NLP, PCX);
+                                    disk2_info->timeout = 1;
+                                }
                             }
+                        } else {
+                            sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                      " READ_DATA: sim_fseek error.\n", PCX);
                         }
 
                         break;
@@ -498,41 +502,49 @@ static uint8 DISK2_Write(const uint32 Addr, uint8 cData)
                             pDrive->track = disk2_info->cyl; /* update track */
                        }
 
-                        sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET);
-                        for(i=0;i<pDrive->nsectors;i++) {
-                            /* Read sector */
-                            file_offset = ftell((pDrive->uptr)->fileref);
-                            rtn = sim_fread(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
-                            if (rtn != 3) {
-                                sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
-                                          " WRITE_DATA: sim_fread error.\n", PCX);
-                            }
-                            if(sdata.u.header[2] == disk2_info->sector) {
-                                if(sdata.u.header[0] != disk2_info->cyl) {
-                                    sim_printf("DISK2: " ADDRESS_FORMAT
-                                           " WRITE_DATA Incorrect header: track" NLP, PCX);
-                                    disk2_info->timeout = 1;
-                                }
-                                if(sdata.u.header[1] != disk2_info->head) {
-                                    sim_printf("DISK2: " ADDRESS_FORMAT
-                                           " WRITE_DATA Incorrect header: head" NLP, PCX);
-                                    disk2_info->timeout = 1;
+                        if(sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET) == 0) {
+                            for(i=0;i<pDrive->nsectors;i++) {
+                                /* Read sector */
+                                file_offset = ftell((pDrive->uptr)->fileref);
+                                rtn = sim_fread(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
+                                if (rtn != 3) {
+                                    sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                              " WRITE_DATA: sim_fread error.\n", PCX);
                                 }
+                                if(sdata.u.header[2] == disk2_info->sector) {
+                                    if(sdata.u.header[0] != disk2_info->cyl) {
+                                        sim_printf("DISK2: " ADDRESS_FORMAT
+                                                   " WRITE_DATA Incorrect header: track" NLP, PCX);
+                                        disk2_info->timeout = 1;
+                                    }
+                                    if(sdata.u.header[1] != disk2_info->head) {
+                                        sim_printf("DISK2: " ADDRESS_FORMAT
+                                                   " WRITE_DATA Incorrect header: head" NLP, PCX);
+                                        disk2_info->timeout = 1;
+                                    }
 
-                                selchan_dma(sdata.u.data, pDrive->sectsize);
-                                sim_fseek((pDrive->uptr)->fileref, file_offset+3, SEEK_SET);
-                                sim_fwrite(sdata.u.data, 1, (pDrive->sectsize), (pDrive->uptr)->fileref);
-                                break;
-                            }
-                            rtn = sim_fread(sdata.raw, 1, pDrive->sectsize, (pDrive->uptr)->fileref);
-                            if (rtn != (size_t)(pDrive->sectsize)) {
-                                sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
-                                          " WRITE_DATA: sim_fread error.\n", PCX);
-                            }
-                            if(i == pDrive->nsectors) {
-                                sim_printf("DISK2: " ADDRESS_FORMAT " Sector not found" NLP, PCX);
-                                disk2_info->timeout = 1;
+                                    selchan_dma(sdata.u.data, pDrive->sectsize);
+                                    if(sim_fseek((pDrive->uptr)->fileref, file_offset+3, SEEK_SET) == 0) {
+                                        sim_fwrite(sdata.u.data, 1, (pDrive->sectsize), (pDrive->uptr)->fileref);
+                                    } else {
+                                        sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                                  " WRITE_DATA: sim_fseek error.\n", PCX);
+                                    }
+                                    break;
+                                }
+                                rtn = sim_fread(sdata.raw, 1, pDrive->sectsize, (pDrive->uptr)->fileref);
+                                if (rtn != (size_t)(pDrive->sectsize)) {
+                                    sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                              " WRITE_DATA: sim_fread error.\n", PCX);
+                                }
+                                if(i == pDrive->nsectors) {
+                                    sim_printf("DISK2: " ADDRESS_FORMAT " Sector not found" NLP, PCX);
+                                    disk2_info->timeout = 1;
+                                }
                             }
+                        } else {
+                            sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                      " WRITE_DATA: sim_fseek error.\n", PCX);
                         }
                         break;
                     case DISK2_CMD_WRITE_HEADER:
@@ -544,8 +556,12 @@ static uint8 DISK2_Write(const uint32 Addr, uint8 cData)
 
                         i = disk2_info->hdr_sector;
                         selchan_dma(sdata.raw, 3);
-                        sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * (pDrive->sectsize + 3) * pDrive->nsectors) + (i * (pDrive->sectsize + 3)), SEEK_SET);
-                        sim_fwrite(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
+                        if(sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * (pDrive->sectsize + 3) * pDrive->nsectors) + (i * (pDrive->sectsize + 3)), SEEK_SET) == 0) {
+                            sim_fwrite(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
+                        } else {
+                            sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                      " WRITE_HEADER Command: sim_fseek error.\n", PCX);
+                        }
 
                         disk2_info->hdr_sector++;
                         if(disk2_info->hdr_sector >= pDrive->nsectors) {
@@ -557,14 +573,18 @@ static uint8 DISK2_Write(const uint32 Addr, uint8 cData)
                         track_offset = pDrive->track * pDrive->nheads * pDrive->nsectors * (pDrive->sectsize + 3);
                         sim_debug(CMD_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
                                   " READ_HEADER Command\n", PCX);
-                        sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET);
-                        rtn = sim_fread(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
-                        if (rtn != 3) {
+                        if(sim_fseek((pDrive->uptr)->fileref, track_offset + (disk2_info->head_sel * pDrive->nsectors * (pDrive->sectsize + 3)), SEEK_SET) == 0) {
+                            rtn = sim_fread(sdata.raw, 1, 3, (pDrive->uptr)->fileref);
+                            if (rtn != 3) {
+                                sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
+                                          " READ_HEADER: sim_fread error.\n", PCX);
+                            }
+                        } else {
                             sim_debug(ERROR_MSG, &disk2_dev, "DISK2: " ADDRESS_FORMAT
-                                      " READ_HEADER: sim_fread error.\n", PCX);
+                                      " READ_HEADER: sim_fseek error.\n", PCX);
                         }
                         selchan_dma(sdata.raw, 3);
-
+                        
                         break;
                     default:
                         sim_printf("DISK2: " ADDRESS_FORMAT " Unknown CMD=%d" NLP, PCX, disk2_info->ctl_op);
diff --git a/AltairZ80/s100_disk3.c b/AltairZ80/s100_disk3.c
index 703a24c5..cf6171c3 100644
--- a/AltairZ80/s100_disk3.c
+++ b/AltairZ80/s100_disk3.c
@@ -546,36 +546,39 @@ static uint8 DISK3_Write(const uint32 Addr, uint8 cData)
 
                 dataBuffer = (uint8 *)malloc(xfr_len);
 
-                sim_fseek((pDrive->uptr)->fileref, file_offset, SEEK_SET);
+                if(sim_fseek((pDrive->uptr)->fileref, file_offset, SEEK_SET) == 0) {
 
-                if(disk3_info->iopb[DISK3_IOPB_ARG1] == 1) { /* Read */
-                    rtn = sim_fread(dataBuffer, 1, xfr_len, (pDrive->uptr)->fileref);
+                    if(disk3_info->iopb[DISK3_IOPB_ARG1] == 1) { /* Read */
+                        rtn = sim_fread(dataBuffer, 1, xfr_len, (pDrive->uptr)->fileref);
 
-                    sim_debug(RD_DATA_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT
-                              "  READ @0x%05x T:%04d/S:%04d/#:%d %s\n",
-                              disk3_info->sel_drive,
-                              PCX,
-                              disk3_info->dma_addr,
-                              pDrive->cur_track,
-                              pDrive->cur_sect,
-                              pDrive->xfr_nsects,
-                              rtn == (size_t)xfr_len ? "OK" : "NOK" );
+                        sim_debug(RD_DATA_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT
+                                  "  READ @0x%05x T:%04d/S:%04d/#:%d %s\n",
+                                  disk3_info->sel_drive,
+                                  PCX,
+                                  disk3_info->dma_addr,
+                                  pDrive->cur_track,
+                                  pDrive->cur_sect,
+                                  pDrive->xfr_nsects,
+                                  rtn == (size_t)xfr_len ? "OK" : "NOK" );
 
 
-                    /* Perform DMA Transfer */
-                    for(xfr_count = 0;xfr_count < xfr_len; xfr_count++) {
-                        PutByteDMA(disk3_info->dma_addr + xfr_count, dataBuffer[xfr_count]);
+                        /* Perform DMA Transfer */
+                        for(xfr_count = 0;xfr_count < xfr_len; xfr_count++) {
+                            PutByteDMA(disk3_info->dma_addr + xfr_count, dataBuffer[xfr_count]);
+                        }
+                    } else { /* Write */
+                        sim_debug(WR_DATA_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT
+                                  " WRITE @0x%05x T:%04d/S:%04d/#:%d\n", disk3_info->sel_drive, PCX, disk3_info->dma_addr, pDrive->cur_track, pDrive->cur_sect, pDrive->xfr_nsects );
+
+                        /* Perform DMA Transfer */
+                        for(xfr_count = 0;xfr_count < xfr_len; xfr_count++) {
+                            dataBuffer[xfr_count] = GetByteDMA(disk3_info->dma_addr + xfr_count);
+                        }
+
+                        sim_fwrite(dataBuffer, 1, xfr_len, (pDrive->uptr)->fileref);
                     }
-                } else { /* Write */
-                    sim_debug(WR_DATA_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT
-                              " WRITE @0x%05x T:%04d/S:%04d/#:%d\n", disk3_info->sel_drive, PCX, disk3_info->dma_addr, pDrive->cur_track, pDrive->cur_sect, pDrive->xfr_nsects );
-
-                    /* Perform DMA Transfer */
-                    for(xfr_count = 0;xfr_count < xfr_len; xfr_count++) {
-                        dataBuffer[xfr_count] = GetByteDMA(disk3_info->dma_addr + xfr_count);
-                    }
-
-                    sim_fwrite(dataBuffer, 1, xfr_len, (pDrive->uptr)->fileref);
+                } else {
+                    sim_debug(ERROR_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT " READWRITE: sim_fseek error.\n", disk3_info->sel_drive, PCX);
                 }
 
                 free(dataBuffer);
@@ -623,8 +626,11 @@ static uint8 DISK3_Write(const uint32 Addr, uint8 cData)
                 fmtBuffer = (uint8 *)malloc(data_len);
                 memset(fmtBuffer, disk3_info->iopb[DISK3_IOPB_ARG2], data_len);
 
-                sim_fseek((pDrive->uptr)->fileref, file_offset, SEEK_SET);
-                sim_fwrite(fmtBuffer, 1, data_len, (pDrive->uptr)->fileref);
+                if(sim_fseek((pDrive->uptr)->fileref, file_offset, SEEK_SET) == 0) {
+                    sim_fwrite(fmtBuffer, 1, data_len, (pDrive->uptr)->fileref);
+                } else {
+                    sim_debug(WR_DATA_MSG, &disk3_dev, "DISK3[%d]: " ADDRESS_FORMAT " FORMAT: sim_fseek error.\n", disk3_info->sel_drive, PCX);
+                }
 
                 free(fmtBuffer);
 
diff --git a/AltairZ80/s100_fif.c b/AltairZ80/s100_fif.c
index fc7dd738..8299ff66 100644
--- a/AltairZ80/s100_fif.c
+++ b/AltairZ80/s100_fif.c
@@ -227,7 +227,7 @@ static int DoDiskOperation(desc_t *dsc, uint8 val)
     }
     current_disk_flags = (fif_dev.units + current_disk) -> flags;
     if ((current_disk_flags & UNIT_ATT) == 0) { /* nothing attached? */
-        if ( (current_disk_flags & UNIT_DSK_VERBOSE) && (warnAttached[current_disk] < warnLevelDSK) ) {
+        if ((current_disk_flags & UNIT_DSK_VERBOSE) && (warnAttached[current_disk] < warnLevelDSK)) {
             warnAttached[current_disk]++;
 /*02*/sim_printf("FIF%i: " ADDRESS_FORMAT " Attempt to select unattached FIF%d - ignored." NLP, current_disk, PCX, current_disk);
         }
@@ -245,20 +245,26 @@ static int DoDiskOperation(desc_t *dsc, uint8 val)
             /*Sleep(250); */
             memset(blanksec, 0, SEC_SZ);
             addr = dsc->track * SPT;
-            sim_fseek(cpx, addr * SEC_SZ, SEEK_SET);
-
-            /* write a track worth of sectors */
-            for (kt=0; kt < SPT; kt++) {
-                sim_fwrite(blanksec, 1, sizeof(blanksec), cpx);
+            if (sim_fseek(cpx, addr * SEC_SZ, SEEK_SET) == 0) {
+                /* write a track worth of sectors */
+                for (kt=0; kt < SPT; kt++) {
+                    sim_fwrite(blanksec, 1, sizeof(blanksec), cpx);
+                }
+            } else {
+                if ((current_disk_flags & UNIT_DSK_VERBOSE) &&
+                    (warnAttached[current_disk] < warnLevelDSK)) {
+                    warnAttached[current_disk]++;
+                    sim_printf("FIF%i: " ADDRESS_FORMAT " sim_fseek error." NLP, current_disk, PCX);
+                }
             }
             break;
 
         case READ_SEC:
             addr = (dsc->track * SPT) + dsc->sector - 1;
-            sim_fseek(cpx, addr * SEC_SZ, SEEK_SET);
+            if (sim_fseek(cpx, addr * SEC_SZ, SEEK_SET) == 0) {
             rtn = sim_fread(blanksec, 1, SEC_SZ, cpx);
-            if ( (rtn != SEC_SZ) && (current_disk_flags & UNIT_DSK_VERBOSE) &&
-                (warnAttached[current_disk] < warnLevelDSK) ) {
+                if ((rtn != SEC_SZ) && (current_disk_flags & UNIT_DSK_VERBOSE) &&
+                    (warnAttached[current_disk] < warnLevelDSK)) {
                 warnAttached[current_disk]++;
                 sim_printf("FIF%i: " ADDRESS_FORMAT " sim_fread error." NLP, current_disk, PCX);
             }
@@ -266,16 +272,30 @@ static int DoDiskOperation(desc_t *dsc, uint8 val)
             for (kt = 0; kt < SEC_SZ; kt++) {
                 PutBYTEWrapper(addr++, blanksec[kt]);
             }
+            } else {
+                if ((current_disk_flags & UNIT_DSK_VERBOSE) &&
+                    (warnAttached[current_disk] < warnLevelDSK)) {
+                    warnAttached[current_disk]++;
+                    sim_printf("FIF%i: " ADDRESS_FORMAT " sim_fseek error." NLP, current_disk, PCX);
+                }
+            }
             break;
 
         case WRITE_SEC:
             addr = (dsc->track * SPT) + dsc->sector - 1;
-            sim_fseek(cpx, addr * SEC_SZ, SEEK_SET);
+            if (sim_fseek(cpx, addr * SEC_SZ, SEEK_SET) == 0) {
             addr = dsc->addr_l + (dsc->addr_h << 8); /* no assumption on endianness */
             for (kt = 0; kt < SEC_SZ; kt++) {
                 blanksec[kt] = GetBYTEWrapper(addr++);
             }
             sim_fwrite(blanksec, 1, SEC_SZ, cpx);
+            } else {
+                if ((current_disk_flags & UNIT_DSK_VERBOSE) &&
+                    (warnAttached[current_disk] < warnLevelDSK)) {
+                    warnAttached[current_disk]++;
+                    sim_printf("FIF%i: " ADDRESS_FORMAT " sim_fseek error." NLP, current_disk, PCX);
+                }
+            }
             break;
 
         default:
diff --git a/AltairZ80/s100_hdc1001.c b/AltairZ80/s100_hdc1001.c
index 2bdde839..ac78d65f 100644
--- a/AltairZ80/s100_hdc1001.c
+++ b/AltairZ80/s100_hdc1001.c
@@ -361,7 +361,7 @@ static uint8 HDC1001_Write(const uint32 Addr, uint8 cData)
 
     switch(Addr & 0x07) {
         case TF_SDH:
-            hdc1001_info->sel_drive = (cData >> 3) & 0x03;
+            hdc1001_info->sel_drive = (cData >> 3) & 0x03; /* intentional fallthrough */
         case TF_DATA:
         case TF_ERROR:
         case TF_SECNT:
diff --git a/AltairZ80/s100_mdsa.c b/AltairZ80/s100_mdsa.c
index 91adbbf5..d2898dc1 100644
--- a/AltairZ80/s100_mdsa.c
+++ b/AltairZ80/s100_mdsa.c
@@ -411,8 +411,11 @@ static uint8 MDSA_Read(const uint32 Addr)
                         if(pDrive->uptr->fileref == NULL) {
                             printf(".fileref is NULL!" NLP);
                         } else {
-                            sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET);
-                            sim_fwrite(sdata.u.data, 1, MDSA_SECTOR_LEN, (pDrive->uptr)->fileref);
+                            if (sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET) == 0) {
+                                sim_fwrite(sdata.u.data, 1, MDSA_SECTOR_LEN, (pDrive->uptr)->fileref);
+                            } else {
+                                printf("%s: sim_fseek error" NLP, __FUNCTION__);
+                            }
                         }
                         break;
                     case IMAGE_TYPE_CPT:
@@ -455,12 +458,16 @@ static uint8 MDSA_Read(const uint32 Addr)
                                 printf(".fileref is NULL!" NLP);
                             } 
                             else {
-                                sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET);
-                                rtn = sim_fread(&sdata.u.data[0], 1, MDSA_SECTOR_LEN,
-                                    (pDrive->uptr)->fileref);
-                                if (rtn != MDSA_SECTOR_LEN) {
+                                if (sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET) == 0) {
+                                    rtn = sim_fread(&sdata.u.data[0], 1, MDSA_SECTOR_LEN,
+                                                    (pDrive->uptr)->fileref);
+                                    if (rtn != MDSA_SECTOR_LEN) {
+                                        sim_debug(ERROR_MSG, &mdsa_dev, "MDSA: " ADDRESS_FORMAT
+                                                  " READ: sim_fread error.\n", PCX);
+                                    }
+                                } else {
                                     sim_debug(ERROR_MSG, &mdsa_dev, "MDSA: " ADDRESS_FORMAT
-                                              " READ: sim_fread error.\n", PCX);
+                                              " READ: sim_fseek error.\n", PCX);
                                 }
                             }
                             break;
diff --git a/AltairZ80/s100_mdsad.c b/AltairZ80/s100_mdsad.c
index 9191641d..580f9342 100644
--- a/AltairZ80/s100_mdsad.c
+++ b/AltairZ80/s100_mdsad.c
@@ -499,9 +499,12 @@ static uint8 MDSAD_Read(const uint32 Addr)
                         if(pDrive->uptr->fileref == NULL) {
                             sim_printf(".fileref is NULL!" NLP);
                         } else {
-                            sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET);
-                            sim_fwrite(sdata.u.data, 1, MDSAD_SECTOR_LEN,
-                                (pDrive->uptr)->fileref);
+                            if (sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET) == 0) {
+                                sim_fwrite(sdata.u.data, 1, MDSAD_SECTOR_LEN,
+                                           (pDrive->uptr)->fileref);
+                            } else {
+                                sim_printf("%s: sim_fseek error" NLP, __FUNCTION__);
+                            }
                         }
                         break;
                     case IMAGE_TYPE_CPT:
@@ -733,13 +736,17 @@ static uint8 MDSAD_Read(const uint32 Addr)
                                     if(pDrive->uptr->fileref == NULL) {
                                         sim_printf(".fileref is NULL!" NLP);
                                     } else {
-                                        sim_fseek((pDrive->uptr)->fileref,
-                                            sec_offset, SEEK_SET);
-                                        rtn = sim_fread(&sdata.u.data[0], 1, MDSAD_SECTOR_LEN,
-                                            (pDrive->uptr)->fileref);
-                                        if (rtn != MDSAD_SECTOR_LEN) {
+                                        if (sim_fseek((pDrive->uptr)->fileref,
+                                                      sec_offset, SEEK_SET) == 0) {
+                                            rtn = sim_fread(&sdata.u.data[0], 1, MDSAD_SECTOR_LEN,
+                                                            (pDrive->uptr)->fileref);
+                                            if (rtn != MDSAD_SECTOR_LEN) {
+                                                sim_debug(ERROR_MSG, &mdsad_dev, "MDSAD: " ADDRESS_FORMAT
+                                                          " READ: sim_fread error.\n", PCX);
+                                            }
+                                        } else {
                                             sim_debug(ERROR_MSG, &mdsad_dev, "MDSAD: " ADDRESS_FORMAT
-                                                      " READ: sim_fread error.\n", PCX);
+                                                      " READ: sim_fseek error.\n", PCX);
                                         }
                                     }
                                     break;
diff --git a/AltairZ80/s100_ss1.c b/AltairZ80/s100_ss1.c
index 2f00503d..032b80a8 100644
--- a/AltairZ80/s100_ss1.c
+++ b/AltairZ80/s100_ss1.c
@@ -304,7 +304,7 @@ static uint8 SS1_Read(const uint32 Addr)
 
     switch(Addr & 0x0F) {
         case SS1_S8259_L:
-            sel_pic = SLAVE_PIC;
+            sel_pic = SLAVE_PIC;    /* intentional fallthrough */
         case SS1_M8259_L:
             if((ss1_pic[sel_pic].OCW3 & 0x03) == 0x03) {
                 cData = ss1_pic[sel_pic].ISR;
@@ -319,7 +319,7 @@ static uint8 SS1_Read(const uint32 Addr)
             }
             break;
         case SS1_S8259_H:
-            sel_pic = SLAVE_PIC;
+            sel_pic = SLAVE_PIC;    /* intentional fallthrough */
         case SS1_M8259_H:
             cData = ss1_pic[sel_pic].IMR;
             sim_debug(PIC_MSG, &ss1_dev, "SS1: " ADDRESS_FORMAT
@@ -437,7 +437,7 @@ static uint8 SS1_Write(const uint32 Addr, uint8 cData)
 
     switch(Addr & 0x0F) {
         case SS1_S8259_L:
-            sel_pic = SLAVE_PIC;
+            sel_pic = SLAVE_PIC;    /* intentional falltrough */
         case SS1_M8259_L:
             if(cData & 0x10) {
                 sim_debug(PIC_MSG, &ss1_dev, "SS1: " ADDRESS_FORMAT
@@ -457,7 +457,7 @@ static uint8 SS1_Write(const uint32 Addr, uint8 cData)
             }
             break;
         case SS1_S8259_H:
-            sel_pic = SLAVE_PIC;
+            sel_pic = SLAVE_PIC;    /* intentional fallthrough */
         case SS1_M8259_H:
             if(ss1_pic[sel_pic].config_cnt == 0) {
                 sim_debug(PIC_MSG, &ss1_dev, "SS1: " ADDRESS_FORMAT " WR: %s PIC IMR=0x%02x.\n", PCX, (sel_pic ? "Slave " : "Master"), cData);
diff --git a/AltairZ80/vfdhd.c b/AltairZ80/vfdhd.c
index 52de6091..ec6d799d 100644
--- a/AltairZ80/vfdhd.c
+++ b/AltairZ80/vfdhd.c
@@ -233,7 +233,7 @@ static t_stat vfdhd_attach(UNIT *uptr, CONST char *cptr)
     unsigned int i = 0;
 
     r = attach_unit(uptr, cptr);                        /* attach unit                          */
-    if ( r != SCPE_OK)                                  /* error?                               */
+    if(r != SCPE_OK)                /* error?                               */
         return r;
 
     /* Determine length of this disk */
@@ -248,6 +248,9 @@ static t_stat vfdhd_attach(UNIT *uptr, CONST char *cptr)
             break;
         }
     }
+    if(i == VFDHD_MAX_DRIVES) {
+        return (SCPE_IERR);
+    }
 
     if(uptr->capac > 0) {
         r = assignDiskType(uptr);
@@ -331,6 +334,9 @@ static t_stat vfdhd_detach(UNIT *uptr)
             break;
         }
     }
+    if(i == VFDHD_MAX_DRIVES) {
+        return (SCPE_IERR);
+    }
 
     DBG_PRINT(("Detach VFDHD%d\n", i));
     r = diskClose(&vfdhd_info->drive[i].imd);
@@ -582,8 +588,7 @@ static void VFDHD_Command(void)
             case IMAGE_TYPE_DSK:
                 if(pDrive->uptr->fileref == NULL) {
                     sim_printf(".fileref is NULL!" NLP);
-                } else {
-                    sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET);
+                } else if(sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET) == 0) {
                     rtn = sim_fread(&sdata.u.sync, 1, 274, /*VFDHD_SECTOR_LEN,*/ (pDrive->uptr)->fileref);
                     if (rtn != 274) {
                         sim_debug(ERROR_MSG, &vfdhd_dev, "VFDHD: " ADDRESS_FORMAT " READ: sim_fread error.\n", PCX);
@@ -596,7 +601,8 @@ static void VFDHD_Command(void)
                     }
 
                     DBG_PRINT(("VFDHD: " ADDRESS_FORMAT " READ: Sync found at offset %d" NLP, PCX, vfdhd_info->datacount));
-
+                } else {
+                    sim_debug(ERROR_MSG, &vfdhd_dev, "VFDHD: " ADDRESS_FORMAT " READ: sim_fseek error.\n", PCX);
                 }
                 break;
             case IMAGE_TYPE_CPT:
@@ -641,17 +647,20 @@ static void VFDHD_Command(void)
                     sim_printf(".fileref is NULL!" NLP);
                 } else {
                     DBG_PRINT(("VFDHD: " ADDRESS_FORMAT " WR drive=%d, track=%d, head=%d, sector=%d" NLP,
-                        PCX,
-                        vfdhd_info->sel_drive,
-                        pDrive->track,
-                        vfdhd_info->head,
-                        vfdhd_info->sector));
-                    sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET);
+                               PCX,
+                               vfdhd_info->sel_drive,
+                               pDrive->track,
+                               vfdhd_info->head,
+                               vfdhd_info->sector));
+                    if(sim_fseek((pDrive->uptr)->fileref, sec_offset, SEEK_SET) == 0) {
 #ifdef USE_VGI
-                    sim_fwrite(&sdata.u.sync, 1, VFDHD_SECTOR_LEN, (pDrive->uptr)->fileref);
+                        sim_fwrite(&sdata.u.sync, 1, VFDHD_SECTOR_LEN, (pDrive->uptr)->fileref);
 #else
-                    sim_fwrite(sdata.u.data, 1, 256, (pDrive->uptr)->fileref);
+                        sim_fwrite(sdata.u.data, 1, 256, (pDrive->uptr)->fileref);
 #endif /* USE_VGI */
+                    } else {
+                        sim_printf("%s: sim_fseek error" NLP, __FUNCTION__);
+                    }
                 }
                 break;
             case IMAGE_TYPE_CPT:
diff --git a/AltairZ80/wd179x.c b/AltairZ80/wd179x.c
index c8937f99..8e339768 100644
--- a/AltairZ80/wd179x.c
+++ b/AltairZ80/wd179x.c
@@ -1112,7 +1112,7 @@ uint8 WD179X_Write(const uint32 Addr, uint8 cData)
                                       " FORMAT T:%d/H:%d/N:%d=%d/L=%d[%d] Fill=0x%02x\n", PCX,
                                       pDrive->track, wd179x_info->fdc_head,
                                       wd179x_info->fdc_fmt_sector_count,
-                                      wd179x_info->fdc_sectormap[wd179x_info->fdc_fmt_sector_count],
+                                      wd179x_info->fdc_sectormap[wd179x_info->fdc_fmt_sector_count - 1],
                                       wd179x_info->fdc_dataindex, wd179x_info->fdc_sec_len, sdata.raw[0]);
 
                             wd179x_info->fdc_gap[1] = 0;