Don't buy an external hard drive... better to buy a "hard drive enclosure". That way, if either the box electronics or the drive goes bad, you can replace for not much money. (I once had a SimpleTech external drive which died.... about 2 months out of warranty... (Bummer!) I literally had to break the box to get to the hard drive. I put the drive into an external enclosure, and all was OK. The external enclosure has more features, too.)
You needn't spend more than $50 on a hard drive enclosure. Check Newegg reviews for one with a high rating.