diff --git a/desktop/desktop_main.s b/desktop/desktop_main.s
index e754337..9fcd2d9 100644
--- a/desktop/desktop_main.s
+++ b/desktop/desktop_main.s
@@ -2076,9 +2076,14 @@ start:
         ;; Restore machine to text state
         sta     ALTZPOFF
         jsr     exit_dhr_mode
+
+        ;; S3D2 /RAM driver still in place?
+        RAMSLOT := DEVADR + $10 + 3*2 ; Slot 3, Drive 2
+        cmp16   RAMSLOT, NODEV
+        beq     quit            ; No, so give up
         jsr     reinstall_ram
 
-        jmp     quit_code_addr
+quit:   jmp     quit_code_addr
 
 fail:   jsr     DESKTOP_SHOW_ALERT
         rts
diff --git a/macros.inc b/macros.inc
index 04702cd..8f50d43 100644
--- a/macros.inc
+++ b/macros.inc
@@ -46,7 +46,11 @@ end:
 ;;; Pad with zeros to the given address
 
 .macro PAD_TO addr
+    .if (addr - *) >= 0
         .res    addr - *, 0
+    .else
+        .error .sprintf("Padding offset %d", addr - *)
+    .endif
 .endmacro
 
 ;;; ============================================================