When you compare how big computers are today to only a few years ago, it's amazing. And when I say 'how big' I don't mean the size as they sit there on your desk. I mean the speed, the size of the hard drives, and how fast their processors are. My first computer many, many years ago, was more like hand held calculator compared to today's models.
Likewise, external storage media has grown proportionally. Only a few years ago, there weren't that many brands in the external hard drive market, but today, there are plenty. Which means if you don't know much about these, it can be difficult to find the right one if you go shopping. Unless you're a complete computer geek, you might end up standing there scratching your head.
It's important to know exactly what you want, and exactly what you want it to do before you go shopping. That way you won't end up buying something that's way too much for what you need, or on the other hand, you won't end up buying something that just doesn't have enough juice. When you figure out exactly what you need, you won't be stuck with these common problems.
The first question is how big is your hard drive, and how many computers you will be using this for. If you have only one hard drive that is 100 GB or so, then your needs are minimum, and you could likely get away with a 500 GB back up system. However, if you have multiple computers, you may need some more horsepower.
Also, consider the type of data you'll be backing up. Is it primary documents, or do you have a lot of music or video or other media that takes up a large amount of space? These effect the writing times, and this can effect which drive is absolutely best for you.
One more thing to think about is how often you plan on backing up your machine. Will you be doing it on a daily basis? Or will you be content to back up your system once a week, or even once a month? These are important questions, and will play a part in determining how much back up you need.
One more thing to think about is portability. Will you be carrying your backup device around with you? Or will it be sitting in a designated location in your home? Portable models and desktop models are different in size and configuration and portable models are generally more expensive.
After you decide all of these questions, then you are much better prepared to hit the stores, online or out there in the world. You'll have a much better idea of exactly what you need, and you'll save time and money as a result. Now get out there and buy yourself a hard drive.