We recently reviewed the Hitachi LifeStudio Desk Plus here on Pocket-lint; an external hard disk drive with plenty of bells and whistles, but if you want it simple, small and fast then the Hitachi Touro Mobile Pro could be more up your street.
Simple, because there's nowt too it apart from a connector port; small - it measures in at 80 x 126 x 15mm; and fast due to the USB 3.0 connectivity.
It comes bundled with an easy to use back up manager program, that will let you replicate your chosen files and folders at the click of a button, and also has a cloud based back-up option too - although you'll need to cough up if you want more space than 3GB.
As for the drive itself, it is available in 500GB and 750GB flavours, and you're getting a USB bus-powered (so no additional power-plug) device that weighs just 155g and is compatible with USB 2.0 as well as the newer standard.
With USB 3.0 its makers claim a transfer rate of up to 5 Gbit/sec; 480 Mbit/sec using USB 2.0. We managed to back up 80GB of mixed size files in around 25 minutes using the USB 3.0 option.
The build quality is decent, if somewhat a little plasticy, but Hitachi definitely has the mobile moniker right. This would fit it your bag or packet without causing much more of a bulge than your smartphone does.
Out now, the Hitachi Touro Mobile Pro 500GB costs around £55, it's £15 or so more for the 750GB version.