A few years ago if you had a gig of storage inside your PC, you would have paid a lot for it.
These days, a gig gets filled up in no time. In fact, three gigs is the low end of many systems, and that's starting to sound small.

v-grab You may be familiar with these sounds: the whine of a spindle motor and the constant chatter of heads looking for data on a hard disk. Some folks complain about the chatter. But compared to the first hard disks, we're spoiled.

"We've gone from one hundred pounds to less than one ounce to store a gigabyte of information," says Currie Muncie, Director of Advanced Storage at IBM's Almaden Research Center.

He says things were a lot different in the '50s, "The first guys -- when they started out to try and make these disks -- they would take an epoxy paint mixture, ground some rust particles into it, put that in a Dixie cup, strain that through a women's nylon to filter it down, and then pour it on a spinning disk and let it spread out as it was spinning, to coat the surface of the disk."

v-grab We've certainly come a long way since then, but the first brown disks were important because they were the first replacements for punch cards.

According to Muncie, the disks "held five megabytes of information which, at the time, replaced sixty thousand punch cards."

But it wasn't the storage capacity of the new disks that was impressive. It was the fact that any record -- the equivalent of one punch card -- could be accessed at random in less than one second. Of course, that five megabyte convenience was about the size of a very large refrigerator and cost about ten thousand dollars per megabyte.

"We had these devices in the data centers," says Muncie. "When we started to get into desktops and workstations, you started to see these smaller form factors."

v-grab But that would take several years. The second wave started in 1973 when IBM introduced a new drive code named Winchester. Today's drives are still based on the technology first used in this Star Trek Enterprise-looking device. Cost? About one hundred dollars per megabyte.

Capacities kept increasing, but the drives didn't get much smaller. In 1981, this one-hundred-pound chainsaw-looking device broke new ground by storing one gig of data.

But in the mid-1980s with personal computers on the horizon, the AT drive broke new ground by packing twenty megabytes into a relatively small size. A computer didn't have to take up a whole room anymore.

v-grab But it wasn't until 1991 with the invention of magneto resistive heads that capacity and speed increased while size kept being reduced. The drives weighed two pounds and cost two dollars per megabyte. "That fundamentally changed our ability and the sensitivity of the head to measure very small features on the surface of the disk," adds Muncie.

That means more and more data could be crammed onto a disk. And because it was closer together, it could be accessed faster.

Just how fast? Well, this is the inside of a disk drive. The arm that is rapidly moving back and forth positions the heads over the surface of the disc, just like the arm of a record player is used to place the needle on a record. To appreciate how fast the slider is moving, compare it geometrically to something we can all relate to like an airplane.

v-grab "If I take the slider, which is moving the head across the disk, and compare that to a 747 aircraft, that's like the 747 flying down the runway where the nose of the aircraft is only twenty millimeters off the surface and the tail is less than two millimeters off the surface of the runway," explains Muncie. "And in terms of speed, it's the equivalent of the aircraft flying at four hundred times the speed of sound."

Muncie says if you do the math, that will get you from New York to San Francisco in less than a minute. The point is these things are really flying, and they continue to get faster and smaller.

 Muncie believes that by the year 2001, this drive -- which is the size of a quarter -- will hold one gigabyte at a cost of two to three cents per megabyte, and it won't require any straining of oxide particles through women's nylons.


Nearly every desktop computer and server in use today contains one or more hard disk drives. Every mainframe and supercomputer is normally connected to hundreds of them. You can even find VCR-type devices and camcorders that use hard disks instead of tape. These billions of hard disks do one thing well - they store changing digital information in a relatively permanent form. They give computers the ability to remember things when the power goes out.

In this edition of How Stuff Works we'll take apart a hard disk so that you can see what's inside, and also discuss how they organize the gigabytes of information they hold in files!

Hard Disk Basics
Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.

At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques described in the HSW article titled How Tape Recorders Work. Hard disks and cassette tapes also share the major benefits of magnetic storage - the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.

Let's look at the big differences between the cassette tapes and hard disks so you can see how they differ:

Because of these differences, a modern hard disk is able to store an amazing amount of information in a small space. A hard disk can also access any of its information in a fraction of a second.

A typical desktop machine will have a hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image. No matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.

There are two ways to measure the performance of a hard disk:

The other important parameter is the capacity of the drive - the number of bytes it can hold.

Inside a Hard Disk
The best way to understand how a hard disk works is to take a look inside. [Note that opening a hard disk ruins it, so this is not something to try at home unless you have a defunct drive.]

Here is a typical hard disk drive:

It is a sealed aluminum box with controller electronics attached to one side. The electronics control the read/write mechanism and the motor that spins the platters. The electronics also assemble the magnetic domains on the drive into bytes (reading) and turn bytes into magnetic domains (writing). The electronics are all contained on a small board that detaches from the rest of the drive:

Underneath the board are the connections for the motor that spins the platters, as well as a highly-filtered vent hole that lets internal and external air pressures equalize:

Removing the cover from the drive reveals an extremely simple but very precise interior:

In this picture you can see:

In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read-write heads:

The mechanism that moves the arms on a hard disk has to be incredibly fast and precise. It can be constructed using a high-speed linear motor.

Many drives use a "voice coil" approach - the same technique used to move the cone of a speaker on your stereo moves the arm.

Storing the Data
Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, which sectors are pie-shaped wedges on a track, like this:

A typical track is shown in yellow, while a typical sector is shown in blue. A sector contains a fixed number of bytes - for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters.

The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes. High-level formatting then writes the file-storage structures like the file allocation table into the sectors. This process prepares the drive to hold files.


There's lots of detailed information on hard disks available on the web. The following links should help you get started.

Formatting and Partitioning

FAT and File Management Hardware IDE and SCSI Interfaces