What Is a Hard Drive and How Does It Work?

The invention of the hard disk drive in 1956 marked a revolutionary leap in data storage technology. Developed by IBM engineer Reynold B. Johnson and his team, the first hard disk drive was known as the IBM Model 350 Disk File, part of the IBM 305 RAMAC computer system. This groundbreaking device utilized magnetic disks for data storage, allowing for rapid access to large amounts of information compared to the sequential access of tape drives.

The original unit was as large as two refrigerators and stored a mere 5 megabytes of data on fifty 24-inch disks. Over the decades, HDDs have evolved dramatically in terms of capacity, size, and speed, significantly shaping the landscape of modern computing and data management.

Hard disks 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 a tape and floppy disk.

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 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.

In the next section, we'll talk about the main differences between cassette tapes and hard disks.

Let's look at the big differences between two types of storage devices — cassette tapes and hard disks:

­Because of these differences, the storage capacity of a modern hard disk is pretty amazing. A hard disk can also access any of its information in a fraction of a second.

Typical desktop computers 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, which is the number of bytes it can hold.

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 disk 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 is used to move the arm.

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

A typical track is shown in yellow; 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.

For more information on hard disks and related topics, check out the links on the next page.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.