for backup to disk devices there should be an automatic cleanup session that runs at 12:00 (DeleteUnprotectedMediaFreq global parameter) but I dont think it works properly. What version of DP are you running?
I set this to run every day in cron on a linux CM....
omnimm -delete_unprotected_media -all
that should work for you!
hope this helps....click KUDOS if this reply is deemed helpful to you! thanks!