I installed the xfce-mount-plugin today after installing xfce and when I click on a usb stick in the popup, it fails to unmount the device. I can unmount it in thunar though.
You have to care that no applications, not even Thunar, has any files or folders open. You might want to check by using "lsof /dev/sdb". Further, you might want to pass the force flag to the unmount command. heck th e"commands" tab in the settings dialog and edit the umount command: sudo umount -f %d HTH.