Buy a Seagate FreeAgent Go Portable Hard Drive 500GB from PC World for £63.87 using promo code 10DRIVES
Once you have your order number, claim your free USB dock & case accessory pack (worth about £16) direct from Seagate, head to http://gopromo.seagateactive.com
(UK residents only, claim until 14th June 2010).
This is for the silver edition but the black one is only 2p more. It is a USB powered hard drive so no need for a separate power supply. Please note that you must use the Promo Code 10DRIVES to get it for this price from PC World, I think the code might work for delivery only.