I really, really, really with this was built in functionality. I believe finding "dead/changed" interfaces has been added to the newest versions of NPM, but not volumes
I've seen this question asked before on the forums, and the only answer I personally have is still mostly a manual process:
1. To find new volumes: To run the network discoveries over every so often and manually add any new volumes found
2. To find volumes that "are no longer there": I have a custom report with this SWQL as the "data source". I run it daily. When the report returns results, I go into "manage nodes" and delete the grey/"no longer present" volume. Usually when a volume disappears from a node a "new one" was added to the node (as Orion sees it, even if it has the same drive letter) so I then I add any new volumes as well. I go to the node and do a "list resources" (may have to "force refresh" as it caches the old resource list) and add any new volumes.
The SWQL query I use is:
SELECT NodeID, VolumeID, Caption,StatusIcon, Type, Responding, FullName, LastSync, VolumeType, VolumeDescription
where Responding = 'N'
and StatusIcon like 'Down%'
order by NodeID