External USB (or FireWire) drive enclosures make it possible to convert a 3.5" desktop hard drive into a semi-portable storage device.
While you wouldn't want to lug one on a plane, they're certainly useful for moving hundreds of gigabytes of data from one computer to another or for making offline backups of drives of equal or lesser size. (Drives have become cheap enough so you can easily buy two and use one to back up the other rather than rely on tape or burning to DVD as a solution.)
One of the many hidden caveats with using external drive enclosures is their power supplies. Most of these enclosures claim they can supply 1.5 amps to 2 amps of power to the device contained within. The exact power rating is usually printed on the underside of the power transformer or "brick" supplied with the unit. If the enclosure has a built-in transformer and plugs directly into a wall outlet, then the power rating should be printed on the back or underside of the unit itself or at least be in the product's documentation.
Desktop hard drives usually require anywhere from 0.45 amps to 0.8 amps to power up, and CD drives require 1.5 amps or more. Many of the cheaper enclosures use less well-designed power supplies that may not perform as rated. If the enclosure's power supply isn't capable of delivering that kind of power consistently, then the drive may behave strangely. It may be able to read reliably, but not burn; it may deliver, for instance, a "power calibration error" message or be visible as a drive, but not be able to read or write at all. Most of them claim to deliver 1.5 or 2 amps, but whether they actually follow through on those claims is another story.
To avoid problems like those, stick with hardware that's either backed by a major manufacturer's name or that has already been tested in other contexts with appropriate hardware. Adaptec, for instance, sells a USB/FireWire hard drive enclosure kit that supplies more than adequate power to its host devices (although it's for hard drives only, not CD/DVD drives).
Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!