http://www.techradar.com/news/best-external-desktop-and-portable-hard-disk-drives

We're storing an increasing volume of important data on our external hard drives, so it's essential you buy the best external hard drive that suits your needs. 

This means making sure you get an external drive with the right amount of capacity - you don't want to run out of space, but nor do you really want to pay for storage you're not going to use.