Seagate FreeAgent Go 640GB 2.5" USB Portable Drive for $104.99 shipped after this coupon.  Note that this coupon is meant for new customers only so you might have to create a new account to use it.  Here is a screenshot for your reference. This 2.5" portable drive does not require a separate power supply and is also smaller, making it more convenient to carry around.  Most places have it for about $129.