How ScanDisk Fixes Bad Sectors (103956)

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 Q103956


When ScanDisk encounters a bad sector it attempts to patch the sector. This is known as hot fixing or hot patching.

NOTE: If you are running in check mode (/CHECKONLY), ScanDisk does not attempt to hot fix the bad sectors.


Hot patching is the process of taking a file that contains one or more unreadable (bad) sectors and recovering all the data that is still readable. Any data that lies in a bad sector is replaced by zeros.

For example, if a file occupies clusters 1, 2, 3, 4, and 5, and cluster 3 contains one or more bad sectors, ScanDisk reads as many of the sectors from cluster 3 as possible and moves the data to a new location (for example cluster 7). Any sectors from cluster 3 that could not be read are replaced with sectors filled with zeros in cluster 7. The FAT chain for the file is updated and cluster 3 is marked bad. After this process is finished, the file occupies clusters 1, 2, 7, 4, and 5.

NOTE: This process makes the file fully readable, but the data that was in the defective sectors is still lost. It is not possible for ScanDisk to recover data from unreadable sectors.

Modification Type: Major Last Reviewed: 11/19/1999
Keywords: KB103956