External drives are one of the marvels of the digital era. Their storage capacity keeps going up, and the price per megabyte keeps going down.

First, a quick introduction to some computer terms.

A "bit" is the basic building block of computer data. It has a value of either "1" or "0."

A "byte" is made up of bits - usually eight of them. A byte represents individual characters: letters, numbers or symbols. In the binary number system, eight bits together as a byte can represent 256 different letters, numbers and symbols. Thus, it takes one byte to make the letter "a."

A "kilobyte" is roughly one thousand bytes (actually 1,024 bytes in normal computer usage). That's about 1,000 letters, numbers or symbols -roughly three paragraphs of text or about two-thirds of an average page in a book.

A "megabyte" is roughly one million bytes (1,048,576 bytes).

A "gigabyte" is roughly one billion bytes (1,073,741,824 bytes).

A "terabyte" is roughly one trillion bytes (1,099,511,627,776 bytes).

"Whoa, Darnay," you might be thinking. "What does all that mean in plain English?"

Well, based on typical file sizes for documents, uncompressed music files, digital photo files and uncompressed movie DVDs, one gigabyte can hold about 256 music files, 341 digital photos or 178,000 pages of documents.

One terabyte can hold about 262,000 music files, 350,000 digital photos, 140 DVD movies or 183 million documents.

Of course, you can store a heck of a lot more music, movies and documents if they're compressed as mp3, divx or zip files.

And I, being the packrat that I am, have a heck of a lot of files.

Which is why I need external drives.

Just a year ago, a 500 gigabyte external storage drive was the biggest I could get for home use, and it set me back several hundred dollars.

Today, I can buy a 1 terabyte drive for less than I paid for that 500 gig drive last year.

So, of course, I bought one (shhh - don't tell my wife).

When I first bought a computer back in the Stone Age (circa early 1980s), I could save stuff on a 5.25 inch, 360 kilobyte floppy diskette.

As data storage space has grown over the years, I've gone from saving my computer stuff on big diskettes to small floppy disks to Zip disks to optical disks to CD writable discs to DVD writable discs to multigig external drives to, now, 1 terabyte externals.

You'd think I'd have enough space now with a terabyte drive.

But I filled it up within two weeks and had to go buy a second one recently (double shhh - really, don't tell my wife).

"Filled it up?" you might be thinking. What in the world do I have to store and save that requires multiple terabyte drives?

Well, a lot of stuff. A lot of computer stuff. Family photos, family videos, audio files, scans, PDFs, professional work, personal work, backups of my music CDs and movie DVDs, backups of the backups.

You know: Important stuff.

Cheap storage is a bad, bad thing for a packrat like me. It just means that there is more stuff I can save in a smaller space.

In the days when you saved papers and files in boxes, even a dozen or so boxes were managable. The pieces of paper probably numbered in the thousands and you could probably do a good "spring cleaning" job on it all over an uninterrupted weekend.

But a terabyte drive can hold millions of documents, thousands of photos, thousands of audio files, hundreds of video files.

You'll need more than a good weekend to go through all this stuff. We're talking months or years here.

So, I've got these terabyte drives I'm filling up and they're in addition to the 24 other external hard drives I have packed in my electronics closet, ranging in sizes from 160 gigs to 500 gigs each.

I'm in trouble. If the average life expectancy of a person is about 78 years, I've reached the crossover point in age and amount of stored data where I will not live long enough to sort through it all.

Even more trouble: The first petabyte external drive is predicted to arrive in the consumer marketplace by 2014.

One petabyte holds 1,000 terabytes.

In 2005, Google estimated the Internet held about 5 million terabytes of data.

Well, look out, Internet - the Darnaynet is catching up.

(Keith Darnay manages the online department and Web site for the University of Mary. His own site, featuring this column going back to 1995, is at www.darnay.com.iec.)