A 320GB, USB 2.0 hard disk I got to replace one of my ailing Maxtor 120GB drives. The casing is metallic (I assume aluminum or a zinc alloy), and dissipates quite a bit of heat (which is good, considering that the drive is on 24/7, plugged into a NSLU2).

Time will tell whether it will stand up to the punishment (although I'm considering ways to keep it cool).