Windows 2000 Backup Not working

When you schedule a Windows 2000 unattended backup by using a standalone tape
drive (library), the first backup job completes successfully. However, after you
change the media, the next scheduled backup job does not back up the desired
folder andfiles. Also, you may receive one of the following error messages in
the backup report log file:

The operation was not performed because no unused media was available.

-or-

Operation was not performed, because the specified media cannot be found.