I originally used an Iomega 120gb hard drive (silver series) that Buy.com said was 4200 rpm. However, after checking Iomega's website, they made the 120gb drive in both 4200 and 5400 rpm. The box gives you no clue, but after trying it out and not getting enough power from the USB connection, I assume its 5400. I connected the second lead to a car usb adapter and it worked exactly as described in the previous post. Then I apparently fried it trying to tap into the 5v connection on the drive.
I then ordered (Ebay) an 80gb version (Model # 33222) of the same drive which only comes in 4200rpm, and it works great so far. It powers up when I unlock the car, so it is ready to go when I start the engine. It doesn't give that pathetic beeping sound described in the next post because it gets enough power from the USB port. It has a couple of times disappeared (stereo switched to FM) and it took a minute before the stereo would recognize it again. I don't know if that has anything to do with the hard drive, because I've occasionally had problems with the stero turning itself off and back on repeatedly for a minute or two after I start it, when I didn't even have anything connected to the USB port.
So far, I'm very happy with the 80gb drive. My entire music collection fits on there easily, and the sound quality is much better than when I connect my Ipod.