Generally speaking don't buy a packaged external hard drive. Getting an enclosure is your best bet. This means you'll actually be able to select the class of the hard drive, its size, and specs via consumer research. Not buying some black box.
The second thing to look into is an external power supply. While this seems bulky, extra cords, etc. It will help extend the life of your drive since you'll be less prone to rapid plug/unplug/plug cycles that are better left for USB drives, not spinning hard drivers, and keep its power up allowing for it to end writes, park heads, close file system, etc. On actual power. Not power derived from the disk slowly spinning down.
