Thanks JoaoMatos for your reply. This is a new tape drive. I'm using a driver from the tape manufacturer, Quantum. The tape itself is brand new but I believe the drive may be a refurbished drive. LiveUpdate doesn't pickup any new updates so I guess I have the latest.
No, I don't have another SCSI card, channel or slot for testing purpose. What is weird is that ntbackup completes successfully.
Definitely sounds like the tape drive is defective. Double check to see if it actually is a new drive or a refurbished one. I was running into the same issue using an HP LTO3 tape drive in a OVERLAND ArchVault12. I used about 3 different cleaning tapes with NO LUCK. Whoever the manufacture is, check if they have any diagnostic software tools to check the drive. I used the HP Storageworks Library and Tape Tools 4.10. Ended up helping alot. I also had a spare single Dell powervault tape drive that I used for testing to verify the hardware issue. Once OVERLAND came out to replace the drive, its been working since. The only issue im having now is the full backup jobs seem to take longer. I've never had any luck using the symantec device drivers but you can try that if you'd like. I'm using Backupexec 12 SP4.
Hope this helps!.