Ive got one of these http://www.amazon.co.uk/New-Trent-Du...ref=pd_cp_ce_1
5000mah: does what it's meant to do, you can charge any electronic device with it (that you already charge from a USB port).
It comes with a nice coiled cable which has a USB connector on one end, and 2 adaptors: mini- and micro-usb.
So you can use it for charging the external battery itself, and for charging the HD from the external battery. You can also use your own USB cables if you want.
Amazon also have an 11,000mah model!
They're much cheaper on Amazon than with the manufacturers, though you may find them even cheaper o ebay or similar. This brand had a lot of positive reviews on Amazon, and it seemed discounted, so I bought it from there.