diff --git a/PDP11/pdp11_rq.c b/PDP11/pdp11_rq.c
index c99b9141..39e8fe52 100644
--- a/PDP11/pdp11_rq.c
+++ b/PDP11/pdp11_rq.c
@@ -172,6 +172,7 @@ extern int32 MMR2;
 #define unit_plug       u4                              /* drive unit plug value */
 #define io_status       u5                              /* io status from callback */
 #define io_complete     u6                              /* io completion flag */
+/* we can re-use filebuf because we don't set UNIT_BUFABLE in flags */
 #define rqxb            filebuf                         /* xfer buffer */
 #define RQ_RMV(u)       ((drv_tab[GET_DTYPE (u->flags)].flgs & RQDF_RMV)? \
                         UF_RMV: 0)
diff --git a/sim_disk.c b/sim_disk.c
index a45ffd0c..4b9d0dfa 100644
--- a/sim_disk.c
+++ b/sim_disk.c
@@ -3380,12 +3380,14 @@ if ((uptr->flags & UNIT_BUF) && (uptr->filebuf)) {
         sim_messagef (SCPE_OK, "%s: writing buffer to file: %s\n", sim_uname (uptr), uptr->filename);
         sim_disk_wrsect (uptr, 0, (uint8 *)uptr->filebuf, NULL, (cap + ctx->sector_size - 1) / ctx->sector_size);
         }
+    if (uptr->flags & UNIT_MUSTBUF) {                   /* dyn alloc? */
+        free (uptr->filebuf);                           /* free buffers */
+        uptr->filebuf = NULL;
+        free (uptr->filebuf2);
+        uptr->filebuf2 = NULL;
+        }
     uptr->flags = uptr->flags & ~UNIT_BUF;
     }
-free (uptr->filebuf);                                   /* free buffers */
-uptr->filebuf = NULL;
-free (uptr->filebuf2);
-uptr->filebuf2 = NULL;
 
 update_disk_footer (uptr);                              /* Update meta data if highwater has changed */