Hello,
Well, in this case Backup Exec could have encountered a timeout period during when the drive or the robotic library would have been thrown offline.
Check for event logs for exact information. Also check the following MS Article which could prove handy in troubleshooting such errors.
How to troubleshoot error messages about Event ID 9 and Event ID 11
http://support.microsoft.com/default.aspx?scid=kb;en-us;314093
Also the following document would help you in resolving such issues if they were severe.
http://seer.support.veritas.com/docs/255501.htm
Additional Information :
For information on the recent VERITAS Backup Exec security vulnerabilities, including links to the downloads for the necessary hotfixes, please refer to the following document:
Patch summary for Security Advisories VX05-001, VX05-002, VX05-003, VX05-005, VX05-006, VX05-007
http://seer.support.veritas.com/docs/277429.htm
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Thanks.