The Help File column in the Oct. 23 Sunday Business section incorrectly defined terabyte. A terabyte of memory equals 1,024 gigabytes. (Published 10/25/2005)

QAfter ripping all my CDs to my computer and saving tons of digital photos, I want to back up those files. Should I get an external hard drive or a DVD burner?

AIt's a trade-off of portability versus capacity versus reliability.

An external hard drive will easily hold every file you've created -- they start at 40 gigabytes and go up to a full terabyte, or 1 million GB -- but you need to keep it near the computer for convenient backups. That means that any catastrophe that destroys your computer (flood, fire, meteorite impact, whatever) can take out the drive as well. Plus, an external hard drive is as vulnerable to mechanical failure as one inside a computer.

DVDs, meanwhile, store 4.7 gigabytes each, although "double-layer" drives on some new computers can fit almost twice as much data on write-once "DVD+R DL" media. So you'll probably need multiple DVDs for a full backup. It also takes longer to burn data to DVD than to copy it to a hard drive.

DVDs, however, are sturdier than hard drives, and keeping a second set of them at a separate location is cheap and easy.

Ideally, you'd use both methods -- keep an external hard drive for weekly backups, then archive your important data to DVD less often. But in the real world, it's much easier to remember to plug in a hard drive once a week.

The auto-complete function in Mac OS X Tiger's Mail program keeps suggesting an old address for a friend. How can I get this program to "forget" that address and only suggest the current one?

Let Mail fill out the incorrect address as it usually does, then hit the Tab key to accept its suggestion. Then hold down the Ctrl key as you click on the address (or right-click it), and select "Remove From Previous Recipients List" from the resulting drop-down menu.

Note to Apple: "Remove From Auto-Complete List" might be a little more intuitive.

-- Rob Pegoraro

Rob Pegoraro attempts to untangle computing conundrums and errant electronics each week. Send questions to The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or rob@twp.com.