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Q127055: How to Cause ScanDisk for Windows to Retest Bad Clusters

Article: Q127055
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:95
Operating System(s): 
Keyword(s): diskmem win95 win98 kbDiskMemory
Last Modified: 11-JUN-2002

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The information in this article applies to:

- Microsoft Windows 95 
- Microsoft Windows 98 
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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:

  Q256986 Description of the Microsoft Windows Registry

SUMMARY
=======

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.

MORE INFORMATION
================

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:

- 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 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.

Additional query words:

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Keywords          : diskmem win95 win98 kbDiskMemory 
Technology        : kbWin95search kbWin98search kbZNotKeyword3 kbWin98
Version           : WINDOWS:95
Issue type        : kbhowto

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