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.
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