Hello,
Stopping and restarting the Backup Exec services, or rebooting the server may bring the device back online, but the device may go offline at some point during another backup.
System exhibiting this behavior should be checked for hardware and specifically SCSI related issues. We suggest you to follow the guidelines mentioned below to resolve this issue.
1. If a driver update is available for the tape device or the controller, try installing it.
2. If a firmware update is available for the tape device or the controller, try installing it.
Please refer to the following technote for details on this issue:
http://support.veritas.com/docs/248089
Thank You,
Shweta