Hello,
Since when does the error occur? Are you using the Advanced open file option? Refer to the following techontes in order to resolve the problem:
A backup job in Backup Exec 9.x or 10.x fails with the error: "Unable to get minimum quiet time window for physical volume" and "The semaphore timeout period has expired" when Advanced Open File Option is used.
http://support.veritas.com/docs/258341
When performing a backup operation, a device "request to space to end of data" or "semaphore timeout" or a "SCSI bus timeout" error is generated.
http://support.veritas.com/docs/190598
If the issue persists, revert with details.
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Regards.