Had this happen once ena want to remember the steps to correct.
From Susan B...
ISSUE #5======Cannot run the SBS Backup Wizard and received this error message
"RSM database is corrupt and cannot be rebuilt. Event 85"
CAUSE--------This issue can occur if the RSM database is corrupt.RESOLUTION--------------
-Rebuild the RSM database by:
1. Stop the RSM Service.
2. Delete the RSM databases located at %SystemRoot%\System32\NtmsData.
3. Restart the RSM Service, this will recreate new, empty RSM Databases.
4. Put a new tape in, or a tape that may be deleted.
5. Go to the "properties" of the disk inside the physical locations. Uncheck 'Enable Drive' Hit apply & then check the 'Enable Drive' again.
6. Right click, and prepare.
This will delete it, write a media label to it, add it to the Free Media Pool.More information:235032 Problems with a Damaged Removable Storage Manager Databasehttp://support.microsoft.com/?id=235032