Q: My computer has a 40 Gigabyte hard drive. What’s a hard drive, and what’s a giga?

A: Maybe you remember the old vinyl record players - mostly 33 rpm. A disk drive is a miniature record player, except that a laser is used instead of a needle, and the whole thing is only about 3-1/2 inches in diameter.

Q: I remember being in a computer room many years ago. It had a cabinet that had a lot of what looked like those 33 rpm records, all stacked up. Was that the same thing?

A: Pretty much. Looked like a juke box, didn’t it? Anyhow, the “records” got smaller and smaller, and moved faster and faster. And the data density (the number of bytes of information that could be stored) rose dramatically.

Q: Where did those first disk drives come from?

A: The technology was invented by IBM scientists back in 1956, more than 50 years ago. The units were really big. And expensive. And heavy. But they did a good job. The disks were 24 inches in diameter. An arm moved in and out, much like the arm on the record player we talked about. And each disk could hold 5 megabytes of information. In the old days that was a lot: 5,000,000 chunks of information. Who could ever need more than that?

Q: Well, who could?

A: Almost everyone. Five megabytes could hold about 5 minutes of MP3 music. Clearly, 5 megabytes is not enough.

Q: What’s wrong with magnetic tape? That will hold a lot of data, too, won’t it?

A: Yes, but accessing data on tape is pretty difficult. And slow. To get at data near the beginning of a tape that has recorded information all the way to the end of the tape takes a lot of time. With the disk drive, it takes only a fraction of a second.

Q: So the later models could hold more data?

A: Absolutely! The capacity increased by about 30% per year for each of the following 35 years. Technology was increasing so fast that in a couple of subsequent years the capacity increased by 100% per year. It is now increasing by about 30% per year.

Q: That adds up, doesn’t it?

A: It sure does. Those first disk drives held about 2,000 bits per square inch. In disk drives today the number is as high as 135 billion bits per square inch. That’s quite an increase.

Q: Have they reached the end of the line?

A: Not at all. People are guessing that in just 7 years the capacity will increase by a factor of 10.

Q: So where are we now?

A: Since a normal disk drive in your computer is about 3-1/2 inches in diameter, it has about 10 times the capacity as a 1-inch diameter disk. That comes to about 1,000 billion bytes. And 1,000 billion is 1 terabyte.

Q: Hold on. What’s a terabyte? And, for that matter, what’s a gigabyte?

A: First comes kilo. That’s 1,000. Then comes mega; that’s 1,000 times as big, or 1,000,000. Then giga, which is 1,000 times as big as a mega, or 1,000,000,000. Finally comes tera, which is 1,000 times as big as giga, or 1,000,000,000,000; we call that trillion.

Q: Can you put this in perspective?

A: Sure. If capacity increases as suggested, a typical hard drive will hold the equivalent of 200 DVDs.

Q: Wow. What comes after tera?

A: Don’t ask.*

* Just for giggles: A terabyte (TB) is 1012. Then comes petabyte (PB): 1015. After that is exabyte (EB): 1018. Then zettabyte (ZB): 1012. And another follows that: yottabyte (YB) 1024. (Source: http://en.wikipedia.org)