Does this work?
resource.dopus.com/viewtopic.php ... ight=eject
I haven't tried it with USB devices (and I dare not try it as my Vista/nForce680i machine seems prone to bluescreening when USB drives are plugged in more than once per reboot, ARGH!!) but it should work, assuming they have an Eject option on their context menus.
You should be able to change the example button to use {filepath} or {sourcepath} (depending on the context that the button is run in) instead of D:\ to make it work with any drive letter.