I have several jobs that use a backup-to-disk folder. The problem I am having is that my exchange jobs using that device fail with a Physical Volume Library Media Not Found error. If I run a test job to the device, it runs without a problem. I followed the steps is KB http://seer.support.veritas.com/docs/269716.htm, but this did not resolve the issue.
Try to create another Backup to disk folder (try different partition) and check whether that works. If you are using external device then create Removable Backup to disk folder. Check following technote,
http://support.veritas.com/docs/253764
If not then please specify the exact error message which you get in Job log.