On Windows 32-bit OSes, I use a tool called Unlocker
http://download.cnet.com/Unlocker/30...-10493998.html
It can tell you what is locking a specific file or drive by right-clicking, and also remove those locks (do that at your own risk of course).
On Windows 64-bin OS, it wouldn't run; but I found lockhuntersetup64.exe that puts a right-click menu item "What is locking this file". I Googled but can't remember where I downloaded it from. But you can easily find many sources by Googling. It will also let you remove the locks.