^ Drives with a rotational speed of 5400rpm will not work, they draw more power at start-up than the USB port will provide and don't initialise. You need a drive with a rotational speed of 4200rpm or slower - the slower the better. Look for portable USB drives that house 1.8" disks to avoid dissappointment. Also iDrive will only index 20,000 tracks. If you put more than this on your hard drive, it will probably only work intermittently.
Now read this thread for all the accumulated experience on this topic: