If i was you i would stay away from the cheap ebay shit tbh,


Something like this gets good reviews and you could use usb external HDD's - (You can get them cheaper than this price though if you look about / wait for an offer)
http://www.amazon.co.uk/dp/B004I5OCS...SIN=B004I5OCSY

The next step up would be a proper dedicated NAS, starting at somthing like :

http://www.ebuyer.com/product/152055


Or you could go mad and spend £500 on a multi disc NAS set up,


Or, go down a slightly different line and buy one of these:

http://www.ebuyer.com/product/236579


Leave it on 24/7, low power, quiet etc etc, can be a dedicated media center left connected to you TV, and also used to set up network shares etc etc


Depends how much you want to spend and how much you want to use it?