Tapes that have been used by other Backupsolutions like NTBackup, CA Arcserve and/or Symantec BackupExec prior to trying to use them with Microsofts Data Protection Manager (DPM) may not be recognized by DPM and also not be deleted / set free by DPM onboard methods.
You may use a small tool called mytape.exe that rumorusly has been written by afaik an unknown Microsoft employee of the DPM Team.
The syntax of mytape.exe is:
mytape.exe "Windows name of Tape drive"
The "Windows name of Tape drive" can be found in DPM:
and added like this to the syntax:
Unfortunatly I am explicitly not allowed to have that tool downloadable here on this site. You may want to ask your MS representative if you might get it it or not.