When Not to Use MS-DOS 5.0 CHKDSK and UNDELETE Commands (80496)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0

This article was previously published under Q80496


The Microsoft MS-DOS CHKDSK and UNDELETE commands dated 04/09/91, which are included in the Microsoft MS-DOS 5 Upgrade, do not work correctly on logical drives that require 256 sectors to store each copy of the file allocation table (FAT). If any of the partitions on your hard disk drive are within the ranges listed in the following section and you do not have version 5.0a of the Microsoft MS-DOS 5 Upgrade, which includes CHKDSK.EXE and UNDELETE.EXE files dated 11/11/91, obtain the new CHKDSK and UNDELETE file.

127 MB - 129 MB
254 MB - 258 MB
508 MB - 516 MB
1018 MB - 1030 MB
2035 MB - 2061 MB

If your FAT occupies 256 sectors, using the CHKDSK.EXE file dated 04/09/91 with the /f parameter ("chkdsk /f" [without the quotation marks]) can result in data loss. Using the UNDELETE.EXE file dated 04/09/91 can lead to unpredictable results.

Microsoft has confirmed this to be a problem in MS-DOS version 5.0. This problem was corrected in MS-DOS version 5.0a.


Drives that require a 256-sector FAT occur when the disk is close to 128 MB, 256 MB, 512 MB, or 1024 MB. The closeness to these limits increases as the drive size increases. For example, "close to 128 MB" is within 0.25 MB of 128 MB and "close to 256 MB" is within 0.74 MB of 256 MB.

Determining If You Need to Install the Updated Files

Drives require a 256-sector FAT if they have partitions within the size ranges listed below. Because the potential problems are serious, these ranges include a margin of safety.

127 MB - 129 MB
254 MB - 258 MB
508 MB - 516 MB
1018 MB - 1030 MB
2035 MB - 2061 MB

You can use the Fdisk program to determine if you have a 256-sector FAT with the following procedure:

    At the MS-DOS command prompt, type the following and press the ENTER key:


    Press 4 on the keyboard to select the Display Partition Information option and press ENTER.
    The primary partition size is listed under the Mbytes heading. If the size falls within any of the ranges specified above, obtain the updated CHKDSK.EXE and UNDELETE.EXE files.
    If you have an extended partition, press the ENTER key to view those partition sizes. If any of the extended partition sizes listed under the Mbytes heading fall within the ranges specified above, obtain the CHKDSK.EXE and UNDELETE.EXE files.
    Press the ESC key twice to exit to the MS-DOS command prompt.

"MS-DOS Functions Quick Reference," Ray Duncan, Microsoft Press, Microsoft Corporation

"QUE DOS Programmer's Reference," Terry Dettman, QUE Corporation

"MS-DOS 5.0 Programmer's Reference," Microsoft Press, Microsoft Corporation, page 315

Modification Type: Major Last Reviewed: 12/3/1999
Keywords: KB80496