Even though this thread is years old, I thought I'd post my solution in case it 
helps anybody else.

I have a similar setup and was wondering how to files from disk using DRM. 
Indeed the key is to process file based volumes in the same way as tapes. I use 
the following commands to delete disk based backups older than 30 days from disk

My 'DB backup Series Expiration Days' is set to '30'

 Set DRMDBBackupexpiredays 30
 SET DRMFILEPROCESS yes


 move drmedia * source=dbbackup enddate=today-30 wherestate=mountable 
tostate=vault

When the backups are older than 30 days they will go into 'vault retrieve' 
status. You can then issue the following command

 move drmedia * source=dbbackup wherestate=VAULTRetrieve tostate=ONSITERetrieve

As the retention period for these volumes has expired, they are deleted from 
disk.

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