If you need a workaround, this script can be used to close all tabs pointing to a drive before dismounting it. I use this myself, as I prefer not to mount things removable (I forget why):
cd /desktop
CloseAllTabs M:\*
"C:\Utils\MountUnmount.vbs"
(MountUnmount.vbs is a script which mounts or dismounts a TrueCrypt drive, depending on whether or not it's mounted already.)