diff --git a/libsrc/geos-cbm/disk/dio_cts.s b/libsrc/geos-cbm/disk/dio_cts.s
index 8be343641..043e1f8d8 100644
--- a/libsrc/geos-cbm/disk/dio_cts.s
+++ b/libsrc/geos-cbm/disk/dio_cts.s
@@ -1,6 +1,6 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
-; 2.7.2001
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-26, Greg King
 ;
 ;
 ; unsigned char __fastcall__ dio_phys_to_log (dhandle_t handle,
@@ -59,7 +59,7 @@ _dio_phys_to_log:
         lda (ptr3),y
         tay
         lda driveType,y
-        and #%00000011          ; this is for RamDrive compatibility
+        and #%00001111          ; remove ramDisk flags
         cmp #DRV_1541
         beq dio_cts1541
         cmp #DRV_1571
@@ -67,7 +67,7 @@ _dio_phys_to_log:
         cmp #DRV_1581
         beq dio_cts1581
             
-        lda #DEV_NOT_FOUND      ; unknown device
+        lda #INCOMPATIBLE       ; unsupported device
         ldx #0
         beq ret
 
@@ -91,7 +91,7 @@ _inv_data:
         lda #INV_TRACK
         .byte $2c
 _inv_hand:
-        lda #INCOMPATIBLE
+        lda #DEV_NOT_FOUND
         ldx #0
         beq ret
 
diff --git a/libsrc/geos-cbm/disk/dio_openclose.s b/libsrc/geos-cbm/disk/dio_openclose.s
index ec5aa8795..72e3e32e9 100644
--- a/libsrc/geos-cbm/disk/dio_openclose.s
+++ b/libsrc/geos-cbm/disk/dio_openclose.s
@@ -1,8 +1,8 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-24, Greg King
 ;
 ; based on Atari version by Christian Groessler
-; 2.7.2001
 ;
 ; dhandle_t     __fastcall__ dio_open  (unsigned char device);
 ; unsigned char __fastcall__ dio_close (dhandle_t handle);
@@ -27,11 +27,13 @@ sectsizetab:
 .code
 
 _dio_open:
-        pha
+        cmp #4
+        bcs _inv_drive
         tax
         lda driveType,x         ; check if there's a device
         beq _inv_drive
         txa
+        pha
         clc
         adc #8                  ; normalize devnum
         sta curDevice
diff --git a/libsrc/geos-cbm/disk/dio_read.s b/libsrc/geos-cbm/disk/dio_read.s
index db46c9b69..ac19f9afa 100644
--- a/libsrc/geos-cbm/disk/dio_read.s
+++ b/libsrc/geos-cbm/disk/dio_read.s
@@ -1,6 +1,6 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
-; 2.7.2001
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-27, Greg King
 ;
 ; this file provides the _dio_read function
 ;
@@ -15,7 +15,9 @@
 
 _dio_read:
         jsr dio_params
+        tay
+        bne err
         jsr ReadBlock
         stx __oserror
         txa
-        rts
+err:    rts
diff --git a/libsrc/geos-cbm/disk/dio_stc.s b/libsrc/geos-cbm/disk/dio_stc.s
index 586e3f3db..7398edb63 100644
--- a/libsrc/geos-cbm/disk/dio_stc.s
+++ b/libsrc/geos-cbm/disk/dio_stc.s
@@ -1,6 +1,6 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
-; 2.7.2001
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-27, Greg King
 ;
 ; unsigned char __fastcall__ dio_log_to_phys (dhandle_t handle,
 ;                                             unsigned *sectnum,        /* input */
@@ -55,7 +55,7 @@ _dio_log_to_phys:
         lda (ptr3),y
         tay 
         lda driveType,y
-        and #%00000011          ; this is for RamDrive compatibility
+        and #%00001111          ; remove ramDisk flags
         cmp #DRV_1541
         beq dio_stc1541
         cmp #DRV_1571
@@ -63,7 +63,7 @@ _dio_log_to_phys:
         cmp #DRV_1581
         beq dio_stc1581
             
-        lda #DEV_NOT_FOUND      ; unknown device
+        lda #INCOMPATIBLE       ; unsupported device
         ldx #0
         beq _ret
 
@@ -86,7 +86,7 @@ _inv_data:
         lda #INV_TRACK
         .byte $2c
 _inv_hand:
-        lda #INCOMPATIBLE
+        lda #DEV_NOT_FOUND
         ldx #0
         beq _ret
 
@@ -102,8 +102,8 @@ _loop41:
         bne _nxt
         lda tmp1
         cmp sectab_1541_l+1,x
-        bcc _found
-_nxt:   inx 
+_nxt:   bcc _found
+        inx
         cpx #35
         bne _loop41
         beq _inv_data
@@ -124,12 +124,11 @@ dio_stc1571:
 ; - fall down to 1541
         lda tmp2
         cmp #>683
-        bne _cnt71
+        bne _if71
         lda tmp1
         cmp #<683
-        bcc dio_stc1541
+_if71:  bcc dio_stc1541
             
-_cnt71:     
         lda tmp1
         sec 
         sbc #<683
@@ -138,6 +137,8 @@ _cnt71:
         sbc #>683
         sta tmp2
         jsr dio_stc1541         ; will fall through here
+        tay
+        bne _ret                ; result beyond track 70
             
         ldy #diopp_track
         lda (ptr1),y
@@ -166,7 +167,7 @@ _sub81: lda tmp1
         sbc #0
         sta tmp2
         inx 
-        cpx #81
+        cpx #80
         bne _loop81
         beq _inv_data
             
diff --git a/libsrc/geos-cbm/disk/dio_write.s b/libsrc/geos-cbm/disk/dio_write.s
index 14267803a..a5f747519 100644
--- a/libsrc/geos-cbm/disk/dio_write.s
+++ b/libsrc/geos-cbm/disk/dio_write.s
@@ -1,6 +1,6 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
-; 2.7.2001
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-27, Greg King
 ;
 ; this file provides the _dio_write function
 ;
@@ -15,5 +15,9 @@
 
 _dio_write:
         jsr dio_params
+        tay
+        bne err
         jsr WriteBlock
         jmp setoserror
+
+err:    rts
diff --git a/libsrc/geos-cbm/disk/dio_writev.s b/libsrc/geos-cbm/disk/dio_writev.s
index 7cb9b029f..9b36ed096 100644
--- a/libsrc/geos-cbm/disk/dio_writev.s
+++ b/libsrc/geos-cbm/disk/dio_writev.s
@@ -1,6 +1,6 @@
 ;
-; Maciej 'YTM/Elysium' Witkowiak
-; 2.7.2001
+; 2001-07-02, Maciej 'YTM/Elysium' Witkowiak
+; 2015-08-27, Greg King
 ;
 ; this file provides the _dio_write function
 ;
@@ -15,7 +15,9 @@
 
 _dio_write_verify:
         jsr dio_params
+        tay
+        bne err
         jsr VerWriteBlock
         stx __oserror
         txa
-        rts
+err:    rts