Windows 2000 Server Backup

I am using Windows 2000 Server Backup and Recovery Tools to do backup of the server hard drive. During the backup process, this error message popped up on the screen :
Drive Error : The device reported an error on a request to read data from media.Error reported : Unknown error
FYI, I am using an internal backup unit using DAT tapes with capacity of 20GB. Tape Drives detected by W2K Device Manager : EXABYTE EXB-89008E000112 SCSI Sequential Device.
The backup failed. I have been searching for solutions but to no avail. Thank you in advance for helping.