(Here's a bonus tip for you before we start: What's the difference between an external hard drive and a portable hard drive? To put it simply, an external hard drive requires an external power source, while a portable hard drive can be powered just by your computer. So if your drive needs to be plugged into an AC outlet to be used, it's an external hard drive. If it doesn't, it's a portable one. While requiring that external power source may seem like a drag, the drive often contains a fan, which will help keep it cool and thus help keep your data safer. The downside, of course, is that you'll need an AC outlet to access your data.)
Tip No. 1: Figure out how much you need and then bump it up to the next level of storage. Yes, portable and external hard drives can be expensive, especially the extra-large-capacity ones. But you will spend less graduating to the next capacity level now than you will buying an entirely new drive later.
Let's say you're not a heavy media consumer. You don't download movies, and you don't even listen to music online. (Yes, I know you're out there.) You do have a computer full of Word and Excel files, however, and you wisely realize that you need a backup location for them. In your case, you may be eyeing the 80GB or 120GB portable hard drives because of their low price points. Step it up to the 250GB and rest assured that you won't have to do this kind of shopping again for a long time.
OK, now let's say you are a heavy media consumer. You only own digital music (CD? What's that?), and you're working on building your own high-def movie library. If this is the case, you're clearly in terabyte territory, and you should go as big as you can. Stepping up now will save you money in the long run, and you'll delay having to get that second (or third, or fourth) drive because you've filled it up.
Do you need to back up multiple computers at the same time? A network-attached storage (NAS) device or a RAID may better suit your needs. Simply speaking, a NAS and a RAID are devices that are designed for maintaining large amounts of data. A NAS is essentially a computer that's only job is storing data (a file server), while a RAID is multiple external hard drives working together in one unit. So if you backing up multiple computers with large amounts of data, you may need to step up to 12TB or 16TB, and you can't get a single external hard drive of that capacity.
Tip No. 2" Get USB 3.0 (also called SuperSpeed USB 3.0). It doesn't matter if your computer isn't currently USB 3.0-capable. You'll eventually have to replace your computer (you know you will), and USB 3.0 ports are now cropping up in the new models and will only continue to do increase in popularity. The portable and external hard drives with USB 3.0 are almost all backward compatible with USB 2.0, so you can stick with 2.0 until you make the leap.
The only reason I would skip USB 3.0 is if you're truly crunched for cash and you only deal with word processing documents. Otherwise, get on the bandwagon and enjoy the SuperSpeed ride.
Tip No. 3. Get some kind of automatic backup. Buying an external or portable hard drive to back up your data is a great first step, but it will be worthless if you don't remember to actually use it. Having automatic backup software will take the burden off you and make sure it gets done. I do suggest regularly checking to ensure that the auto backup is working, especially at first.
The only downside to automatic backup is that it can slow down your computer's performance. If you have it set to activate when your computer starts up, for example, your computer may act sluggish until the backup has completed. If you have a large amount of files to back up, this could easily take 30 minutes or more. One way to avoid this is to set your external hard drive to back up at the end of the day, or at another time you know you won't be using your computer.
If none of these scenarios are practical for you, take a look at 10 Ways to Remember To Back Up (although No. 1 won't really pertain to you).