These drives spin down when not active, even when you have System Preferencs set to not let that happen. This causes a problem with both Spotlight and Time Machine. The first Time Machine backup works fine, but the second fails because the drive doesn't spin up before Time Machine times out on it. And more backups always fail because Spotlight has hung on the device at the same time. You have to reboot to clear this.
I contacted G-Technology and they told me to buy software that keeps the drive spinning by constantly using it, even when it isn't needed. There is no settings or jumpers you can change to resolve this. There are a couple of posts on the Apple Support Forums that discuss this as well.
I give it three stars because its an otherwise usable drive for normal use when you're sitting at the machine. But don't buy this if all you want it to do it Time Machine backups.