I have been getting emails with some great deals on external hard drives. However, many are USB 3 only. They do not play with older USB 2 or USB 1 computers, Make sure your computer does USB 3 before grabbing one of those deals.
And do not get one containing its own backup software. Use one of the image backups like EaseUS Free. Search for a Portable Hard Drive instead of an external and be sure it does USB 2 as well as USB 3. If your computer is less than about ten years old, it has USB 2.