This is another device I would look to buy. It is a SlimEx Onyx 40GB USB HDD. It weighs less then 3 oz and measures just 3.9" x 2.4" x 0.4" which is small enough to comfortbly carry around in your pocket. It is a USB 2.0 device which gives it a nice fast transfer rate. It does not require drivers as it's a plug and play device and has a nice magnesium alloy casing making it tough against a few knocks here and there (although I do not recommend that 
)
Via: UberGizmo