Any of the following could cause that problem:
- Another instance of VSS already running against the target volumes
- The target volume may no longer be available and therefore inaccessible
- On-Access anti-virus activity locking up the files CommVault is trying to target
Recommended Antivirus Exclusions for Windows: http://documentation.commvault.com/commvault/v11/article?p=white_paper/security/antivirus_exclusions.htm
- A pending restart (reboot) of the machine:
Also, search through the Windows Application and System Event logs and see whethere there is something odd from when this problem started.
You can also attach a copy of the clBackup here for review.