mal som tento problem, pomohlo toto:

Unable to log on if the boot partition drive letter has changed

CAUSE
This problem can occur if your Windows 2000 boot partition drive letter does not match the drive letter assigned during the initial Windows 2000 Setup. Windows 2000 maintains a record of drive letters in a registry based database and re-assigns drive letters based on Globally Unique Identifiers (GUID) recorded for each volume. Should the volume GUID change or be duplicated (by hard drive cloning software), the original drive letter may not be re-assigned to the boot volume.

This may also occur if you break a system/boot mirror, and then attempt to boot to the old shadow drive if the original primary drive is missing or inaccessible. This is because the volume GUID for the shadow drive is different than that of the original primary drive and does not get the same drive letter assigned.

http://support.microsoft.com/?kbid=249321