I thought you were working with physical usb volumes...my fault.
You mean pgp virtual volumes right?
In that case, windows threat volume as physical.
Lets say you create pgp virtual disk on E:
You can access from My computer.
If you shutdown services, you still are able to access (I could)
if you run mountvol E: /D , disk will be unmounted, and cannot be accessed....however it will still show Mounted in PGP.
PGP does not offer any command to unmount the virtual disk. This is coded into the SDK.
You can also "graphically" automate the action using AutoHotKey, thought.
I hope that helps...