Scheduled NT Backup Hangs

I'm running NT4.0 Server sp6a, using AT to schedule an Exchange 5.5 server
backup every night. I'm using the append option, and changing tapes every 4
days. Sometimes I forget to change tapes, and the tape fills up.
At this point, the ntbackup session hangs, and the only way to recover is to
re-boot the server. (task manager end process gives error message- access
denied) Is there any other way to recover from this?
Thank you.