Hard disks are a commodity product nowadays: the market is flooded with a cornucopia of options, most of them cheap and easy to install. So which is right for you?
The trick is this: find out who makes the actual hard drive inside the external enclosure (that is, inside the nicely-designed plastic or metal box that sits on or under your desk).
It's what's inside that counts, and we'll explore after the jump.
Graphic designers I know swear by their LaCie drives, for example, which use component disks manufactured by Western Digital. Western Digital, Seagate, and Hitachi typically have good reputations among the small group of IT people I talk with. Your mileage may vary, of course: I had a new Seagate disk give me terrible drama just a few months ago.
A well-designed enclosure is more than just eye-candy, too: you want to find one that offers plenty of thermal protection (like vents and even small fans). Fact is, semi-pros can buy an enclosure separately, and replace the less-expensive "guts" when it's time to upgrade. CoolMax makes a great enclosure that I use every day: the CD-311. It has all the connections I could possibly want, and I can even hook up my MacBook's hard drive to it.
Most external HD manufacturers offer a variety of connection options, such as USB 2.0, FireWire 400, FireWire 800, and (more recently) eSATA. For a Mac, typically, FireWire 800 is the fastest connection method -- if your computer supports it (aluminum iMacs, MBPs and Mac Pros do). FireWire 400 ranks second, followed by USB 2.0. Even though USB 2.0 has a faster rated transfer speed (480 Mbits/s versus 400 Mbits/s for FireWire), many Mac users have found that FireWire has a faster sustained throughput than USB 2.0.
eSATA drives are compatible with your Mac, but only if you have an eSATA adapter. Installing one isn't difficult, but it's beyond the Mac 101 bailiwick. Apple doesn't include built-in eSATA ports on new Macs yet. eSATA offers a connection that's over four times faster than FireWire 800.
Perhaps the most important thing to remember is that whatever you buy, chances are it will start giving you trouble in about four years. As someone who has personally witnessed dozens of hard disks give their dying last breath, hard drives are consumable storage just like CDs and DVDs. They wear out over time, and it's best to replace them before problems arise. In fact, for mission-critical data, some IT pros recommend replacing hard disks as often as once every 12 months.
Backing up data is important, but backing up data to a reliable device is golden.
(Full disclosure: Iomega, a manufacturer of external hard disks, was a client of mine from 2002 to 2007. Iomega drives use both Hitachi and Seagate components.)