MS-DOS FORMAT Does Not Preserve Clusters Marked Bad (103548)

The information in this article applies to:
    Microsoft MS-DOS operating system 3.1
    Microsoft MS-DOS operating system 3.2
    Microsoft MS-DOS operating system 3.21
    Microsoft MS-DOS operating system 3.3
    Microsoft MS-DOS operating system 3.3a
    Microsoft MS-DOS operating system 4.0
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.22
    Microsoft Windows 95

This article was previously published under Q103548


Versions of the FORMAT command in versions of MS-DOS earlier than 6.2 do not preserve bad cluster markers in the file allocation table (FAT).

For added safety, run a third-party surface scan program after you run FORMAT or upgrade to MS-DOS version 6.2.


Any hard disk may a have a few bad sectors. Disk surface scan programs, such as Microsoft MS-DOS ScanDisk, Symantec's Norton Utilities Norton Disk Doctor (NDD.EXE), and Central Point Software DiskFix can detect (and mark as bad) sectors that are marginally reliable. This is done by marking the cluster that contains the bad sector with a special entry in the file allocation table (FAT)--FF7h in 12-bit FATs and FFF7h in 16-bit FATs.

Versions of MS-DOS FORMAT earlier than 6.2 do not preserve entries marked as bad in the FAT. Instead, FORMAT clears the FAT, performs its own disk integrity test, and marks clusters with unreliable sectors as bad. Since the FORMAT integrity test is not as thorough as some surface scan programs, and because sector failures can be intermittent, FORMAT may not detect bad sectors that were previously marked as bad by a surface scan utility.

MS-DOS 6.2 FORMAT preserves the FAT entries that are marked as bad (rather than marking them good and retesting them). This reduces the risk of a marginally reliable sector being marked as usable.

In some circumstances, it is desirable to not preserve the bad cluster markers. For example, an errant program may damage the FAT and fill in entries with inappropriate bad cluster markers. In such a case, you can use the FORMAT /C command. The /C switch directs FORMAT to clear the FAT (ignoring bad clusters markers) and revert to its pre-MS-DOS 6.2 behavior. Note, you should run Microsoft ScanDisk or a third-party surface scan program after performing a FORMAT /C command.

WARNING: If you are experiencing increasing problems with bad sectors on your disk, be sure to perform frequent backups of your data and run ScanDisk (or a similar disk utility) regularly. If problems persist consult your hardware vendor.

Modification Type: Major Last Reviewed: 5/6/2003
Keywords: KB103548