Answer Geek: Hard Disk Drive Basics

<br> -- Q U E S T I O N: I recently bought a new hard drive and installed it myself. After loading the EZ files to format the hard drive, a warning message appeared about bad clusters. When I selected the option button to “fix it,” to my surprise it found 185 bad clusters. Does that mean the hard drive I bought is bad?

One more question. What are cylinders, heads, and sectors?

— Thomas

A N S W E R: Hmm … Cylinders? Heads? Sectors? Bad clusters? Thomas, I think what we need here is an introductory lesson in what a hard disk drive is and how it works. Then we’ll take a quick stab at assessing the viability of the new one you are trying to stick into your PC.

But before we do either of those things, let’s pause for a moment to pay respects to that often unappreciated piece of hardware, the hard disk drive. These days, you can hardly buy a desktop machine at any price with a hard disk drive that holds less than 30 gigabytes of data, and the day when a 100 GB hard drive is standard issue is fast approaching. That is a phenomenal amount of storage space! When the first PC hit the shelves some 20 years ago, it came loaded with a hard disk drive that offered all of 10 megabytes of storage. Back then, the cost per MB of storage space was in the neighborhood of $100. Today, it’s down around one penny. So we’re talking a one million percent improvement in less than 20 years.

Pretty impressive, wouldn’t you say?

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Don’t Try This at Home

So what is a hard drive, exactly? If for some crazy reason you decided to take the hard drive out of your PC and monkey around with it — warning, don’t do it! — you’d find a tightly sealed aluminum box with a circuit-board nestled into one side.

The rest of the box is taken up largely with the mechanical devices that do the actual work. Most prominent is a set of mirror-like disks called platters, probably made of aluminum and coated on both sides with a very thin layer of magnetic recording material much like the long strip of plastic “tape” in a cassette tape. Each side holds tens of billions of bits of data.