Disconnect the drive and open the Registry Editior (Start-Run-Regedit).
Navigate to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Enum\V1394
Check under that key and see if you can find anything about your
firewire hard drive. If you do find a key, right click on it and change
the permissions to Allow full control. Now right click and delete the
key. Reboot and try plugging the device in again.