TechReviewSource has a guide on buying an external HDD, you can check it out over here.
With the increasing amount of data storage our lives require these days, it makes sense to invest in an external hard drive. External hard drives promise almost unlimited storage: For under $100, you can add a terabyte of data to your PC or Mac, portable or desktop. That's enough for over 750,000 MP3s or photos, or over 230 DVD-sized movies. We tell you what to consider when choosing the right one for you.