Answer Geek: Hard Disk Drive Basics

-- 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?

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.

The platters are stacked on a spindle that is attached to a motor, which rotates the platters at very high speeds — 3,600 RPM and 7,200 RPM are standard, and there are even drives that spin at speeds of 10,000 and 15,000 RPM. A thin, rather delicate arm extends across each platter. At the tip of each arm is a head, an electromagnetic device that can both record data onto the disk and read the data that is already there.

A typical arrangement would be three platters, six heads, and four arms: one arm above the top platter, one below the bottom, one between the top and middle platters, and one between the bottom and middle platters. The arms, which are mounted in a single stack and move as a group, are attached to a device called an actuator, which controls their movement. The arms flash across the platters at startling speeds, making the trip from one point to another dozens of times per second.

Not Vinyl Records, Tree Rings

So where are we? Let’s see … We’ve covered platters, spindles, arms, heads, and actuators. If you stay with me a bit longer, we’re actually going to get to the question about bad clusters momentarily.

In order to make it easier to keep track of all that data that clutters up your hard drive, the platter is divided up into sections called tracks and sectors. Tracks are concentric circles — if you are trying to picture this, think of the rings of a tree rather than the spiral of a vinyl record. A platter is packed with tens of thousands of these rings. Tracks are then divided into sectors, which typically hold 512 bytes of data, plus some additional data that the computer uses to locate and manage the data. Each track has thousands of sectors.

To understand what a cylinder is, let’s go back for a minute to the typical arrangement of three platters and four arms, and remember that the arms move as a single unit. Suppose that the arm is instructed to move to track number 562. Each platter has two track 562s, one on top and one on the bottom: with three disks there would be six tracks with the number 562, each one directly above or below another. Together they form a virtual cylinder.

That leaves clusters. The track is the largest division on a platter — it holds too much data be an efficient storage unit. The sector is the smallest division, and it is too small to be practical. That’s why we have clusters, which are groups of sectors that are treated as a single entity for purposes of storing data. Also called allocation units, they vary in size, depending on the volume of the hard disk. The PC I am working on at the moment, which is a couple years old, has clusters that are 4,096 bytes in size, indicating each one is made up of eight sectors. All told, my hard disk drive has 623,728 clusters.

Bad Clusters And What They Mean

Now, Thomas, my guess is that the hard drive you are installing is a lot bigger than the measly 2.6 GB device in my machine. So that means you’ve probably got many times the 600,000 clusters that my hard drive has. As a percentage, 185 bad clusters is nothing to panic about. In fact, having a few lost clusters is pretty normal — though they can accumulate precipitously over time; for example, when your machine crashes and a file isn’t saved properly. On the other hand, the fact that you had bad clusters pop up when you formatted the drive may indicate that there are physical defects on the surfaces of the platters.

All in all, I’d say that the drive you bought is probably perfectly fine. However, it might be a good idea to run a disk scanning utility on a regular basis — there is almost certainly one included as part of your operating system — and if more bad clusters pop up, or if you start seeing bad sector warnings, than back up your data pronto, and check your warranty, because you may be looking at an imminent hard drive failure.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.