An attempt to prepare the storage group '<storage group name>' for backup failed because the storage group is already in the process of being backed up. The error code is <error code>. (Note that if a backup was recently aborted, then it may take several minutes for the system to detect the aborted backup and initiate backup cleanup procedures, so this message may be generated if an attempt was made to backup a storage group before a previous backup attempt had fully terminated).
This event is logged when a VSS backup operation fails in Exchange 2007, because:
1. VSS uses the Microsoft Jet database engine to freeze or thaw a snapshot backup.
2. The Jet database engine encounters an error.
3. The Jet database engine silently frees all storage groups from the snapshot session when the error occurs.
See ME930800 to resolve this problem.