We have 3 clients with exactly the same problem. Their ntbackup backs up all their data but gives the error "The requested media failed to mount. The operation was aborted." just before it starts with the system state backup. The end result is that it shows that the backup failed, but we were able to restore files from the backup so it is working up to the system state backup.
All of the clients have HP DLT VS160 backup drives, except one who has a VS80.
If you look on the Microsoft site they say that this happens when your Exchange database is on your system drive. In my case the exchange files are all on drive E:.