CHKDSK Reports "Probable Non-DOS Disk" (101061)

The information in this article applies to:
    Microsoft MS-DOS operating system 6.0

This article was previously published under Q101061


When you run CHKDSK on a logical MS-DOS or DoubleSpace-compressed drive, you may receive the following error message if the drive was not partitioned or formatted with MS-DOS utilities, or if the media descriptor byte in the file allocation table (FAT) for the drive is incorrect or corrupted:

Probable non-DOS disk
Continue (Y/N)?


This error message may occur for one of the following reasons:
    The disk was not formatted with the MS-DOS FORMAT command or was partitioned using a nonstandard partitioning scheme. The CHKDSK command should be used only on MS-DOS disks. Confirm that the disk or drive is an MS-DOS disk and that it is formatted.

    Some disks not formatted with the MS-DOS FORMAT command may cause the CHKDSK command to display this message; however, the CHKDSK process may still function properly. If you have a nonstandard partitioning scheme, make sure the correct device drivers are loaded.
    The media descriptor byte in one or both of the file allocation tables (FATs) for the drive producing the error message is incorrect or corrupted.

    If the drive has been partitioned and formatted with MS-DOS, you can use a disk editor (such a Norton DiskEdit) to edit the FAT. The FAT should be viewed as hexidecimal. The first byte in the FAT for a hard (fixed) disk should be "F8" (without the quotation marks). The following is an example of the first three lines of the FAT and the location of the media descriptor byte:
          00000000:  F8 FF FF FF 03 00 04 00 - 05 00 06 00 07 00 08 00
          00000010:  09 00 0A 00 0B 00 0C 00 - 0D 00 0E 00 0F 00 10 00
          00000020:  11 00 12 00 13 00 14 00 - 15 00 FF FF 17 00 18 00
    WARNING: The above text is an example. You should NOT use a disk editor to change your FAT to match the above text.

    The media descriptor byte is the first two digits after the colon (:) in the first line, in this example. "F8." This is the correct setting for all MS-DOS formatted fixed disks (hard disk drives). Below is an abridged list of valid entries for the media descriptor byte and the media type the descriptor denotes:
          F8     Fixed disk
          F0     3.5-inch, 2-sided, 18-sector
          F9     3.5-inch, 2-sided, 9-sector
          F9     5.25-inch, 2-sided, 15-sector
          FC     5.25-inch, 1-sided, 9-sector
          FD     5.25-inch, 2-sided, 9-sector
          FE     5.25-inch, 1-sided, 8-sector
          FF     5.25-inch, 2-sided, 8-sector
          FE     8-inch, 1-sided, single-density
          FD     8-inch, 2-sided, single-density
          FE     8-inch, 2-sided, double-density
          FD     8-inch, 2-sided, double-density


"The MS-DOS Encyclopedia," Microsoft Press, 1988

Modification Type: Major Last Reviewed: 11/19/1999
Keywords: KB101061