backup failure in windows 2000 server

Hi,

I am manually taking backup of my data. I am mapping the share and
talking backup. Whenever the Ntbackup is prompting for next tape to
continue backup,
it fails and gives the error in backup log and event manager.

On backup log the error is:

"Backup started on 5/30/2006 at 4:44 AM.

Error: The device reported an error on a request to read data from
media.
Error reported: Invalid command.

Backup completed on 5/30/2006 at 6:31 AM.
Directories: 53
Files: 32831
Bytes: 11,062,354,374
Time: 1 hour, 0 minutes, and 9 seconds"

and Event manager gives :

Event Type: Error
Event Source: NTBackup
Event Category: None
Event ID: 8001
Date: 5/30/2006
Time: 6:31:29 AM
User: N/A
Computer: JUPITER
Description:
End Backup of 'Z:' Verify: Off Mode: Append Type:
Normal


Appreciate if someone has any solution.

Thanks

Saket