diff --git a/scp.c b/scp.c
index 53dd118c..a83561a6 100644
--- a/scp.c
+++ b/scp.c
@@ -3656,12 +3656,15 @@ if (dptr == NULL)                                       /* found dev? */
     return SCPE_NXDEV;
 if (uptr == NULL)                                       /* valid unit? */
     return SCPE_NXUN;
-if ((uptr->flags & UNIT_ATT) &&                         /* already attached? */
-    !(uptr->dynflags & UNIT_ATTMULT)) {                 /* and only single attachable */
-    r = scp_detach_unit (dptr, uptr);                   /* detach it */
-    if (r != SCPE_OK)                                   /* error? */
-        return r;
-    }
+if (uptr->flags & UNIT_ATT)                             /* already attached? */
+    if (!(uptr->dynflags & UNIT_ATTMULT) &&             /* and only single attachable */
+        !(dptr->flags & DEV_DONTAUTO)) {                /* and auto detachable */
+        r = scp_detach_unit (dptr, uptr);               /* detach it */
+        if (r != SCPE_OK)                               /* error? */
+            return r;
+        }
+    else
+        return SCPE_ALATT;                              /* Already attached */
 sim_trim_endspc (cptr);                                 /* trim trailing spc */
 return scp_attach_unit (dptr, uptr, cptr);              /* attach */
 }
diff --git a/sim_defs.h b/sim_defs.h
index 48503a3f..b2e25b81 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -384,6 +384,7 @@ struct sim_device {
 #define DEV_V_TYPE      4                               /* Attach type */
 #define DEV_S_TYPE      3                               /* Width of Type Field */
 #define DEV_V_SECTORS   7                               /* Unit Capacity is in 512byte sectors */
+#define DEV_V_DONTAUTO  8                               /* Do not auto detach already attached units */
 #define DEV_V_UF_31     12                              /* user flags, V3.1 */
 #define DEV_V_UF        16                              /* user flags */
 #define DEV_V_RSV       31                              /* reserved */
@@ -393,6 +394,7 @@ struct sim_device {
 #define DEV_DYNM        (1 << DEV_V_DYNM)               /* device requires call on msize routine to change memory size */
 #define DEV_DEBUG       (1 << DEV_V_DEBUG)              /* device supports SET DEBUG command */
 #define DEV_SECTORS     (1 << DEV_V_SECTORS)            /* capacity is 512 byte sectors */
+#define DEV_DONTAUTO    (1 << DEV_V_DONTAUTO)           /* Do not auto detach already attached units */
 #define DEV_NET         0                               /* Deprecated - meaningless */