Hi,
This problem generally occurs due to hardware or some SCSI issue.
Follow the steps given below to resolve the issue.
1) Download and install latest Veritas driver from following link.
http://support.veritas.com/docs/2724747
2) Clean the drive with cleaning tape. Try using different tape (new tape).
3) Try performing erase operation.
4) Download and install latest firmware from backup device manufacturer's web site.
5) Stop "Removable storage" service and change startup type to manual.
6) Install OEM drivers, stop Backup Exec services.
Try backing up with Windows NT and 2000 native backup utility. Observe the results.
If you encounter similar errors in Windows NT and 2000 native backup utility then you may need to replace/repair the drive.Following technote might be useful for you.
How to troubleshoot issues with the "Robotic Library" and "Tape Drives" in VERITAS Backup Exec (tm) 9.x for Windows Servers
http://seer.support.veritas.com/docs/255501.htm