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 and filenames.

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 cause problems:
    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 uncompressed drives.

    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

Modification Type: Major Last Reviewed: 11/15/1999
Keywords: KB111050