In discussion with local CommVault staff, I used the browser-based Admin GUI to delete the disk from the list of available Storage Pools. This appears to have deleted the DDB associated with that disk. Next I tried to delete the disk from the list of Storage Targets but got the error "The mount path has valid backup data".
I traced the problem back to a storage policy named "CommServeDR" that was set up prior to me starting to work with CommVault. I don't know what this is but it was easy enough to update the Primary copy to the new HyperScale disk. This backup runs every hour and the problem is that it has a 7 day retention which means I won't be able to remove my old disk until some time next week.
If any other customers read this, I would recommend opening a ticket with CommVault support. This is certainly not as easy a process as I had originally expected.
Winston, I appreciate the detailed answer you provided and will mark it as the solution.