We have a couple of defects / known issues reported against this sort of problem.
There is also a customer education issue relating to DLM in that DLM is setup to stop you killing the last good backup by accident even if the backup set has reached an expiry date. As such amongst other things it will not erase the only complete backup set for a given selection, and it will not erase a backup set that has dependent sets that cannot be erased yet (for instance full backuop cannot be erased until the last Incremental backup linked with the full can be erased.)
You should perhaps read up on DLM here:
https://www-secure.symantec.com/connect/blogs/data-lifecycle-management-be-2012
With regards the known issues if the disk storage is cartridge based (RDX etc) then the disks may fill up if GRT backups are being performed. We have a private update (Orphan) available to solve this but as it is not a public fix you must log a formal support case to get it (http://www.symantec.com/business/support/index?page=content&id=TECH192382) Note that document also tells you how to manually delete backup sets from the console
We also have a known issue relating to USB disks not erasing data if the backup set on the disk reaches its expiry date when the USB disk is disconnected (which might also affect NAS storage if the network is unreliable) http://www.symantec.com/docs/TECH196201 Running an inventory and catalog job after reconnecting the USB drive can fix this issue.
