By default, ScanDisk for Windows (Scandskw.exe) does not test clusters that are marked as bad in the file allocation table (FAT). This behavior applies for both compressed and uncompressed drives and is consistent with the behavior of the MS-DOS ScanDisk utility (Scandisk.exe) included with MS-DOS version 6.x and Windows 95.

You can configure ScanDisk for Windows to test clusters marked as bad in the FAT by editing the registry. This article describes this procedure and discusses why you might want to clear existing bad cluster marks in the FAT.

WARNING: This procedure could possibly prevent certain applications from functioning properly, prevent certain applications from running at all, or, in rare cases, cause partial to complete data loss. Use the procedure described below only at your own risk.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

ScanDisk for Windows does not normally attempt to repair clusters marked as bad (FF7h for a 12-bit FAT and FFF7h for a 16-bit FAT) because there are many instances in which the clusters have been marked as bad for a valid reason. If a bad cluster mark is cleared in this situation, problems could arise. The following situations are examples of how clearing a bad cluster mark can cause problems:

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Some encryption programs and programs that are copy protected mark one or more clusters as bad to prevent other programs from accessing them. Removing the bad cluster mark could cause problems with these types of applications.

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Clusters marked as bad may indicate legitimate media defects on the physical surface of the disk as identified by another surface scan utility. ScanDisk for Windows may not detect anything wrong with a marginal cluster that was marked as bad by another utility (such as the MS-DOS ScanDisk utility). NOTE: Bad cluster markings due to physical media defects should appear only on uncompressed drives. If ScanDisk is unable to read a cluster on a DriveSpace compressed drive, it will erase the Microsoft DriveSpace FAT (MDFAT) entry for that cluster and change the portion of the file that was using that cluster to zero bytes in length. The physical sectors previously used by the unreadable cluster are marked as free and can be used again.

While many clusters are marked as bad for valid reasons, some disk utilities that are not designed to be run on DriveSpace compressed drives may incorrectly mark a FAT cluster as bad if they are unable to read the data for the compressed cluster. There may also be cases where a cluster is incorrectly marked as bad on an uncompressed drive.

You can force ScanDisk for Windows to retest bad clusters by modifying the value for the following registry key:

   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ 
   Applets\Check Drive\Settings
				

This registry key will exist only if ScanDisk for Windows has already been run. To have ScanDisk test clusters that have been marked as bad, modify the Settings value so that the last two digits are "04." For example, if the current value for Settings is

   B1 03 40 00
				

change it to read:

   B1 03 40 04
				

Once you modify the value for Settings, ScanDisk for Windows will retest any clusters marked as bad. If ScanDisk can correctly read a bad cluster, the following message will be displayed:

   ScanDisk found an error on <volume label> (<drive>)

   Cluster nnn is marked as bad, but ScanDisk cannot detect anything
   wrong with it. Bad clusters are physical areas of your disk that a
   program has identified as being unreliable for storing data.

    - Leave the cluster marked bad and continue. (Default)
      Ensures that the cluster will remain unavailable for storing data,
      which is the safest option. Some encryption programs, or programs
      that are copy protected, mark one or more clusters as bad to
      prevent other programs from accessing them. If you are using one
      of these programs, you should leave the bad cluster mark intact.

    - Clear the bad cluster mark.
      Marks the cluster available for storing data. Although ScanDisk
      did not detect anything wrong with this cluster, you may encounter
      errors if you use it.

    - Test the cluster again.
      Retests the bad cluster. If the test fails, ScanDisk will leave
      the cluster marked as bad and will continue searching your disk
      for other errors. If the test succeeds several times, it is
      probably safe to store data in this cluster.
				

It is important to remember that bad cluster marks on an uncompressed drive may indicate marginal media that cannot be read reliably. Letting ScanDisk for Windows clear a bad cluster mark made by another surface scan utility could result in partial or complete data loss if the cluster cannot be read reliably. Make sure to back up your hard disk before you let ScanDisk repair a bad cluster mark on an uncompressed drive.