This article was previously published under Q127055
IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986 Description of the Microsoft Windows Registry
By default, ScanDisk for Windows (Scandskw.exe) does not test clusters thatare marked as bad in the file allocation table (FAT). This behavior appliesfor both compressed and uncompressed drives and is consistent with thebehavior of the MS-DOS ScanDisk utility (Scandisk.exe) included with MS-DOSversion 6.x and Windows 95.
You can configure ScanDisk for Windows to test clusters marked as bad inthe FAT by editing the registry. This article describes this procedure anddiscusses why you might want to clear existing bad cluster marks in theFAT.
WARNING: This procedure could possibly prevent certain applications fromfunctioning properly, prevent certain applications from running at all, or,in rare cases, cause partial to complete data loss. Use the proceduredescribed 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 asbad (FF7h for a 12-bit FAT and FFF7h for a 16-bit FAT) because there aremany instances in which the clusters have been marked as bad for a validreason. If a bad cluster mark is cleared in this situation, problems couldarise. The following situations are examples of how clearing a bad clustermark can cause problems:
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.
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 diskutilities that are not designed to be run on DriveSpace compressed drivesmay incorrectly mark a FAT cluster as bad if they are unable to read thedata for the compressed cluster. There may also be cases where a cluster isincorrectly marked as bad on an uncompressed drive.
You can force ScanDisk for Windows to retest bad clusters by modifying thevalue for the following registry key:
This registry key will exist only if ScanDisk for Windows has already beenrun. To have ScanDisk test clusters that have been marked as bad, modifythe Settings value so that the last two digits are "04." For example, ifthe 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 retestany 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 drivemay indicate marginal media that cannot be read reliably. Letting ScanDiskfor Windows clear a bad cluster mark made by another surface scan utilitycould result in partial or complete data loss if the cluster cannot be readreliably. Make sure to back up your hard disk before you let ScanDiskrepair a bad cluster mark on an uncompressed drive.