Have you tried looking at what is still open on the drive, what is holding onto it?

Download Process Explorer:
http://technet.microsoft.com/en-us/sysinternals/bb896653

Extract and run it. When running, press Ctrl + F to open the Find Handle or DLL…
Type the drive letter and a colon : in and it should show what is keeping it open.

So for a drive labeled F, you would enter F:

I have had mixed results from this method, sometimes nothing, others there is something…either way, it could help.