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MS-DOS FORMAT Does Not Preserve Clusters Marked Bad

This article was previously published under Q103548
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Versions of the FORMAT command in versions of MS-DOS earlier than 6.2 donot preserve bad cluster markers in the file allocation table (FAT).

For added safety, run a third-party surface scan program after you runFORMAT or upgrade to MS-DOS version 6.2.
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Any hard disk may a have a few bad sectors. Disk surface scanprograms, such as Microsoft MS-DOS ScanDisk, Symantec's NortonUtilities Norton Disk Doctor (NDD.EXE), and Central Point SoftwareDiskFix can detect (and mark as bad) sectors that are marginallyreliable. This is done by marking the cluster that contains the badsector with a special entry in the file allocation table (FAT)--FF7hin 12-bit FATs and FFF7h in 16-bit FATs.

Versions of MS-DOS FORMAT earlier than 6.2 do not preserve entriesmarked as bad in the FAT. Instead, FORMAT clears the FAT, performs itsown disk integrity test, and marks clusters with unreliable sectors asbad. Since the FORMAT integrity test is not as thorough as somesurface scan programs, and because sector failures can beintermittent, FORMAT may not detect bad sectors that were previouslymarked 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 therisk of a marginally reliable sector being marked as usable.

In some circumstances, it is desirable to not preserve the bad clustermarkers. For example, an errant program may damage the FAT and fill inentries with inappropriate bad cluster markers. In such a case, youcan use the FORMAT /C command. The /C switch directs FORMAT to clearthe FAT (ignoring bad clusters markers) and revert to its pre-MS-DOS6.2 behavior. Note, you should run Microsoft ScanDisk or a third-partysurface scan program after performing a FORMAT /C command.

WARNING: If you are experiencing increasing problems with bad sectorson your disk, be sure to perform frequent backups of your data and runScanDisk (or a similar disk utility) regularly. If problems persistconsult your hardware vendor.
6.00 6.20 crc data error reading writing drive

Article ID: 103548 - Last Review: 10/26/2013 13:08:00 - Revision: 3.0

Microsoft MS-DOS 4.0 Standard Edition, Microsoft MS-DOS 5.0 Standard Edition, Microsoft MS-DOS 6.0 Standard Edition, Microsoft MS-DOS 6.22 Standard Edition, Microsoft Windows 95

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