This is a great price for a 500GB portable HD. £60 seems to be about average for a drive of this capacity. That seems to be the usual price for this model too, as it was on Ebuyer before today. This price is only on the black model - the other colours are still around the £60 mark.
I've no idea if it's a mis-price, but maybe the release of the USB3 model of this drive has caused the price drop. It still seems to be £60 in most other places for now though. 252 in stock at time of writing.