Every so often when I try to safely remove a USB drive from my Windows machine I get the Problem Ejecting USB Mass Storage Device error even though I'm sure no programs are using the drive.
Microsoft suggests using a free program to fix this - Process Explorer (from a company called 'Sysinternals' that was purchased by Microsoft a few years back). Once you've downloaded the software it is a simple enough process, but does have the potential to make you system unstable, so make sure you understand the risks before continuing through this quick tutorial.