You don't have to buy an external hard drive; you can roll your own. External drive enclosures are an economical alternative to commercial external hard drives, which sometimes sell at a high premium over the cost of a bare hard drive. These enclosures, most of which cost $20 to $40, accept standard ATA or SATA drives and provide internal power and data connectors for the drive. They also include ATA-to-USB and/or ATA-to-FireWire interface circuitry and an external jack or cable by which the enclosure can be connected to a PC. Models that accept 2.5" notebook hard drives are powered by the USB cable (usually a USB Y-cable, which draws power from two USB ports) or a 6-wire FireWire cable. Models that accept 3.5" hard drives use a separate power brick to supply the higher current needed by standard ATA/SATA hard drives.
Installing a hard drive in one of these enclosures is easy: you simply open the enclosure, secure the hard drive with the supplied mounting screws, connect the internal power and data cables to the drive, and put the cover back on the enclosure. Most enclosures use rubber shock mounting and other means to protect the drive if the enclosure is dropped.
One advantage to rolling your own is that commercial external drives usually have only a one-year warranty. If you build your own external drive with an enclosure and a standard hard drive, the drive has its standard warranty, which may be as long as five years.
External drive enclosures are made by Belkin, IOGEAR, Kingwin, ThermalTake, Vantec, and others, and are widely available online and at big-box retailers. Price is a good indicator of quality. The $20 units we've seen appear fragile and shoddily made. The $30 and $40 products use more metal and less plastic, and appear to be considerably more reliable. The better units sometimes include a cooling fan, which may improve the reliability and service life of the drive.
Some external enclosures can also accept a DVD writer or other drive that uses removable media. If you mount a DVD writer in such an enclosure, make sure the power brick is rated to supply the peak amperage required by the drive, which may be considerably higher than the draw of a hard drive.