If you are sure that there are no save sets in this volume (verify with mminfo), let me suggest that you relabel it. This will delete all save set files. Then unmount the volume and delete it.    
I have an old Disk Volume that was used for some old backups jobs that are no longer needed. It's reporting 41TB, but I've expired all of the save sets and purged them with nsrim -X. Now the volume says no save sets, cleaning is running on the data domain, but the volume is still showing up as 41TB in the Networker Mgmt Console. What's the best practice to safely delete this volume?
Can I safely delete Disk Volume with no backup sets?