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_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