The dark secret about hard drives is that no matter how much storage space you start with, you'll probably need more before very long.

Western Digital My Passport Essential WDME1600TN
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It's a consequence of living in a digital age. Not only do we have data files containing our papers, letters, spreadsheets, and e-mails, but now we also store an unending amount of music, pictures, and even video on our computers. And always -- always -- just when you thought you had everything you needed tucked neatly on your hard drive, up pops something new that you also must have and keep forever.

Thankfully, while it's not infinite, hard drive capacity is scalable. You can add more. You could pop the lid on your PC, dig in with a screwdriver, and add drives until you run out of bays or ports. Very few of us have enough bays or ports to accommodate hard disks labeled up through "Z." Besides, if the inside of your computer is nice and neat, you'll mess it up. If it's already a mess in there, do you really want to deal with it?

Enter the external hard drive.

There's no fuss, no muss, no diving into your computer and tangling yourself in cables. With rare exception, they are true plug-and-play devices that will let you expand your storage capacity with as many drives as you have spare drive letters to apply to them.

The only real downside is that most add a power cable to your configuration so that if you attach four external hard drives you'll also need four free AC outlets. (The smallest members of the group, in terms of capacity and physical size, often eschew AC and work directly off your computer's USB port -- although they often require connection to two ports for sufficient power.)