ScanDisk Does Not Correct Clusters Marked as Bad (111050)
The information in this article applies to:
• Microsoft MS-DOS operating system 6.2
• Microsoft MS-DOS operating system 6.22
This article was previously published under Q111050
This information applies to both Microsoft DoubleSpace and Microsoft
DriveSpace. For MS-DOS 6.22, use DRVSPACE in place of DBLSPACE for commands
If ScanDisk finds clusters that are marked as "bad" in the file allocation
table (FAT) while performing a surface scan, it designates them with a "B"
in the surface-scan map. However, it does not correct or repair the bad
clusters. This is by design and applies when you scan both compressed and
uncompressed drives. Additionally, ScanDisk never marks a cluster as bad on
a DoubleSpace-compressed drive.
By design, ScanDisk does not attempt to repair clusters marked as bad
because there are instances in which these clusters are marked bad for a
valid reason. In these situations, removing the bad cluster marking (FFF7h
or FF7h in the FAT) could cause problems.
NOTE: FF7h is used to mark bad clusters on drives with a 12-bit FAT, while
a 16-bit FAT uses FFF7h. MS-DOS determines the FAT size based on the number
of clusters (allocation units) on the host drive. If there are 4085 or
fewer clusters, a 12-bit FAT is used. If there are 4086 or more clusters, a
16-bit FAT is used.
Below are some examples of how repairing a cluster marked as bad could
• Some applications (such as copy-protected software) mark a cluster as
bad and store data in this cluster. Removing the bad cluster marking
could cause problems with these applications.
• Clusters marked as bad in the FAT may indicate media defects on the
physical disk surface. In this situation, the bad cluster markings in
the FAT were probably put there by a surface-scan utility. Bad
cluster markings due to physical media defects should only appear on
A cluster that is unreadable on a DoubleSpace-compressed drive
most likely does not indicate a media defect on the disk's surface.
ScanDisk never marks a cluster on a compressed drive as bad. If
ScanDisk finds an unreadable cluster on a compressed drive, it will
erase the Microsoft DoubleSpace FAT (MDFAT) entry for that cluster,
and the portion of the file that was using that cluster is
changed to zero bytes in length. The sectors previously being used by
the unreadable cluster are marked as free and can be used again.
ScanDisk marks bad clusters only on the host drive. These bad clusters
correspond to physical defects on the disk's surface.
• A disk utility program that is not DoubleSpace-aware may mark a
cluster as bad on a DoubleSpace drive if it is unable to read data from
the compressed cluster. Contact the manufacturer of that disk utility
program for information on why the cluster is marked as bad and how
to resolve this issue.
NOTE: Microsoft does not support running third-party disk utilities
on DoubleSpace-compressed drives. Use only those utilities included
with MS-DOS 6.2 (ScanDisk, Defrag, and so forth) on compressed drives,
or contact the manufacturer of your disk utility software for
information on DoubleSpace compatibility.
"The MS-DOS Encyclopedia," published by Microsoft Press
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