If you have space inside the case then an internal hard drive is the sensible way to go unless you absolutely need the portability of an external drive. Accessing files and saving them will be vastly quicker than any USB connected external drive though if you have an e-sata connection and external drive the difference will not be so great. Just look on ebuyer and see that you can get a Samsung 1Tb eco for £60 or the faster 7200 rpm for £73. Externals tend to be a bit dearer because of the need for a casing. Personally I would try to find one with it's own power supply rather than running off the USB.