chelms
You should try cleaning your drive with a dedicated cleaning tape. Also try to use the Veritas driver.
Depending on what SCSI controller you have, try to disable sync/wide negotiation, drop transfer at ID drive is set to, say, 10.
Also try to long erase the tape prior of using it for a backup.
If you still get the error try this in the registry:
Modify registry entries HKLM\Software\Veritas\Backup Exec\Engine\Tape Format\FsUseAsyncio from 1 to O
HKLM\Software\Veritas\Backup Exec\Engine\Tape Format\Use Fast Append to 0
Tilo