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A laptop hard drive

Hard drives

Most people are amazed when they discover they can store hundreds of CDs worth of music on an iPod digital music player no bigger than a pack of cards. The original iPod was not much more than a hard drive: an incredibly efficient computer memory device that uses simple magnetism to store vast amounts of information. Hard drives were invented over 50 years ago and have been used in personal computers since the mid-1980s (though flash memory, in so-called solid-state drives, or SSDs, has replaced them in many machines over recent years). The microprocessor in your computer is the bit that does all the "thinking" and calculating—but it's the hard drive that gives your computer its prodigious memory and lets you store digital photos, music files, and text documents. How does it work? Let's take a closer look!

Photo: A 30GB (gigabyte) hard drive from an old laptop computer. The rows of gold pins on the left side are the IDE (Integrated Drive Electronics) connector, which is how the drive plugs into the motherboard of a computer. Newer hard drives use a different kind of connection, SATA (Serial Advanced Technolgoy Attachment), which allows faster data transfer.

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  1. How to store information with magnetism
  2. What are the parts in a hard drive?
  3. Reading and writing data
  4. Who invented the hard drive?
  5. Hard drives and SSDs compared
  6. Find out more

How to store information with magnetism

The science of magnetism is complex. But if you've ever fooled around with a magnet and some nails, you'll know that the technology—the science in action—is quite simple. Iron nails start off unmagnetized but, if you rub a magnet back and forth over them, you can make them magnetic so they stick to one another. Magnetism has some simple, practical uses. For example, junkyards use electromagnets (huge magnets that can be switched on and off with electricity) to pick up and move around piles of metal scrap.

Horseshoe magnet

Photo: Magnets—the technology behind hard drives really is this simple!

Magnetism has another very important use. Suppose you need to leave a message for a friend and all you have is a magnet and an unmagnetized iron nail. Suppose the message is a very simple one: either you will see your friend later that day or not. You could arrange with your friend that you will drop a nail through their letterbox. If the nail is magnetized, it means you will see them later; if the nail is unmagnetized, you won't. Your friend gets in from school and finds a nail on the doormat. They take it to the kitchen table and try to pick up a paperclip. If the clip attaches to the magnet, it must be magnetized—and it must mean you plan to see them later. It's a pretty weird way to leave a message for someone, but it illustrates something very important: magnetism can be used to store information.

If your computer has a 20 gigabyte (GB) hard drive, or you have a 20 GB iPod or MP3 player, it's a bit like a box containing 160 thousand million microscopically small iron nails, each of which can store one tiny piece of information called a bit. A bit is a binary digit—either a number zero or a number one. In computers, numbers are stored not as decimal (base-10) but as patterns of binary digits instead. For example, the decimal number 382 is stored as the binary number 101111110. Letters and other characters can also be stored as binary numbers. Thus, computers store a capital letter A as the decimal number 65 or the binary number 1000001. Suppose you want to store the number 1000001 in your computer in that big box of iron nails. You need to find a row of seven unused nails. You magnetize the first one (to store a 1), leave the next five demagnetized (to store five zeros), and magnetize the last one (to store a 1).

How a hard drive works

In your computer's hard drive, there aren't really any iron nails. There's just a large shiny, circular "plate" of magnetic material called a platter, divided into billions of tiny areas. Each one of those areas can be independently magnetized (to store a 1) or demagnetized (to store a 0). Magnetism is used in computer storage because it goes on storing information even when the power is switched off. If you magnetize a nail, it stays magnetized until you demagnetize it. In much the same way, the computerized information (or data) stored in your PC hard drive or iPod stays there even when you switch the power off.

What are the parts in a hard drive?

A hard drive has only a few basic parts. There are one or more shiny silver platters where information is stored magnetically, there's an arm mechanism that moves a tiny magnet called a read-write head back and forth over the platters to record or store information, and there's an electronic circuit to control everything and act as a link between the hard drive and the rest of your computer.

After a hard-drive crash last year, I was left with an old drive that no longer worked. I took a peek inside, and here's what I found...

The parts/components inside a hard drive

  1. Actuator that moves the read-write arm. In older hard drives, the actuators were stepper motors. In most modern hard drives, voice coils are used instead. As their name suggests, these are simple electromagnets, working rather like the moving coils that make sounds in loudspeakers. They position the read-write arm more quickly, precisely, and reliably than stepper motors and are less sensitive to problems such as temperature variations. Here's another view of the actuator in close-up:

    Closeup of the read/write actuator arm on a hard drive.

  2. Read-write arm swings read-write head back and forth across platter.
  3. Central spindle allows platter to rotate at high speed.
  4. Magnetic platter stores information in binary form.
  5. Plug connections link hard drive to circuit board in personal computer.
  6. Read-write head is a tiny magnet on the end of the read-write arm.
  7. Circuit board on underside controls the flow of data to and from the platter.
  8. Flexible connector carries data from circuit board to read-write head and platter.
  9. Small spindle allows read-write arm to swing across platter.

iPod PCMCIA hard drive and laptop hard drive (outside view). iPod PCMCIA hard drive and laptop hard drive (inside view).
Photo: Little and large: Here's the 30GB laptop hard-drive (shown in the other photos on this page) next to a 20GB PCMCIA hard drive from an iPod. The two drives look strikingly similar and work exactly the same way (both are made by Toshiba), but the iPod drive is even more of a miracle of miniaturization! The green-blue circuit board you can see in the first photo includes the disk controller, a circuit that allows the computer to operate the drive's mechanisms and read/write data to and from it.

The platters are the most important parts of a hard drive. As the name suggests, they are disks made from a hard material such as glass, ceramic, or aluminum, which is coated with a thin layer of metal that can be magnetized or demagnetized. A small hard drive typically has only one platter, but each side of it has a magnetic coating. Bigger drives have a series of platters stacked on a central spindle, with a small gap in between them. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them.

There are two read-write heads for each platter, one to read the top surface and one to read the bottom, so a hard drive that has five platters (say) would need ten separate read-write heads. The read-write heads are mounted on an electrically controlled arm that moves from the center of the drive to the outer edge and back again. To reduce wear and tear, they don't actually touch the platter: there's a layer of fluid or air between the head and the platter surface.

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Reading and writing data

The most important thing about memory is not being able to store information but being able to find it later. Imagine storing a magnetized iron nail in a pile of 1.6 million million identical nails and you'll have some idea how much trouble your computer would get into if it didn't use a very methodical way of filing its information.

When your computer stores data on its hard drive, it doesn't just throw magnetized nails into a box, all jumbled up together. The data is stored in a very orderly pattern on each platter. Bits of data are arranged in concentric, circular paths called tracks. Each track is broken up into smaller areas called sectors. Part of the hard drive stores a map of sectors that have already been used up and others that are still free. (In Windows, this map is called the File Allocation Table or FAT.) When the computer wants to store new information, it takes a look at the map to find some free sectors. Then it instructs the read-write head to move across the platter to exactly the right location and store the data there. To read information, the same process runs in reverse.

How does an electronic computer manipulate all the mechanical nitty gritty in a hard drive? There is an interface (a connecting piece of equipment) between them called a controller. This is a small circuit that operates the actuators, selects specific tracks for reading and writing, and converts parallel streams of data going from the computer into serial streams of data being written to the disk (and vice versa). Controllers are either built into the disk drive's own circuit board or part of the computer's main board (motherboard).

With so much information stored in such a tiny amount of space, a hard drive is a remarkable piece of engineering. That brings benefits (such as being able to store 500 CDs on your iPod)—but drawbacks too. One of them is that hard drives can go wrong if they get dirt or dust inside them. A tiny piece of dust can make the read-write head bounce up and down, crashing into the platter and damaging its magnetic material. This is known as a disk crash (or head crash) and it can (though it doesn't always) cause the loss of all the information on a hard drive. A disk crash usually occurs out of the blue, without any warning. That's why you should always keep backup copies of your important documents and files, either on another hard drive, on a compact disc (CD) or DVD, or on a flash memory stick.

The read-write head on a hard drive.

Photo: The read-write head on a hard-drive. Above) The actuator arm swings the head back and forth so it's in the right position on the drive. Below) Only the tiny extreme end part of the hard drive actually reads from and writes to the platter. Bear in mind that half of what you're seeing in this photo is a reflection in the shiny hard drive surface!

Closeup of the read-write head on a hard drive.

Who invented the hard drive?

IBM 1311 DASD hard drive unit from the 1960s and 1970s

Photo: A classic IBM 1311 "DASD" hard drive unit, manufactured from 1962 until 1975. Each one of these could store about 2 megabytes! Photo by Arnold Reinhold published on Wikimedia Commons under a Creative Commons (CC BY-SA 3.0) licence.

Like many innovations in 20th-century computing, hard drives were invented at IBM as a way to give computers a rapidly accessible "random-access" memory. The trouble with other computer memory devices, like punched cards and reels of magnetic tape, is that they can only be accessed serially (in order, from beginning to end), so if the bit of data you want to retrieve is somewhere in the middle of your tape, you have to read or scan through the entire thing, fairly slowly, to find the thing you want. Everything is much faster with a hard drive, which can move its read-write head very quickly from one part of the disk to another; any part of the disk can be accessed as easily as any other part. The first hard drive was developed by IBM's Reynold B. Johnson and announced on September 4, 1956 as the IBM 350 Disk Storage Unit.

IBM engineers also pioneered floppy disks, which were removable magnetic disks packed in robust plastic cases (originally 20cm or 8in in diameter and wrapped in flexible plastic sleeves; later 133mm or 5.25in in diameter and packed in tough plastic cases). Developed by IBM's Warren Dalziel in 1967 and first sold in 1971, they became hugely popular in microcomputers (the forerunners of PCs) in the late 1970s and early 1980s, but are now obsolete. With a storage capacity of only 1.44MB, they've been completely superseded by USB flash "drives" that offer hundreds or thousands of times more memory in a tiny plastic stick a fraction the size.


IBM engineers developed this groundbreaking magnetic memory (which, in IBM-speak, was called the DASD, pronounced "das-dee"), through a process of continuous improvement from the early 1950s onward and were awarded their final patent on the design in 1970. You can see that the basic read-write mechanism is exactly the same as in today's drives: there are multiple platters (light blue) made up of individual sectors (dark blue) that can be written to or read from by multiple read-write heads (red) mounted on the ends of sliding actuators (orange). The platters are spun by a pulley and motor (green), while the actuators are driven by gears and a motor (yellow). The main difference between this drive and a modern one is the amazing amount of intricate machinery this one contained (which you can read all about in the original patent).

Artwork showing the platters and read head from IBM's original DASD hard drive patent, US3,503,060.

Artwork: The original hard drive. From US Patent 3,503,060: Direct access magnetic disc storage device by William Goddard and John Lynott, IBM Corporation, March 24, 1970, courtesy of US Patent and Trademark Office, with colors added for clarity.

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Hard drives and SSDs compared

Hard drives are tried and tested, high-capacity, and cheap, but they have plenty of drawbacks too. One issue is the amount of time it takes for the read-write head to get itself to the right part of the disk to access the information you want. The heft of a hard drive and its relatively heavy power consumption are also problems, especially in mobile devices such as tablets and smartphones. Reliability is another issue. As you'll have gathered from what you've just read, a hard drive is a wonderful bit of precision engineering with plenty of intricate moving parts. It could easily work for 20 years with no problems at all. Then again, if you've ever suffered a hard-drive head crash (a serious mechanical breakdown caused by something like dirt on one of your hard-drive platters or a sudden mechanical shock), and lost everything you've ever stored on your computer, that's no reassurance: you'll know a hard drive will instantly fall out of love with you if you treat it with less care than it deserves.

Hard drive with RAM memory chip board on top

Photo: SSD drives made with memory chips (above) are replacing hard drives (below).

All these problems—weight, power consumption, access times, and reliability—can be solved by using solid-state drives (SSDs), which typically use flash memory chips instead of spinning magnetic platters. Computer makers have been moving away from hard drives, and toward SSDs, for at least the last decade, largely driven by the trend away from desktop computers and toward mobile devices. Apple iPods are a good example of how times have changed. The original "Classic" iPods, launched in 2001, are little more than hard drives, sound cards, and batteries (you can see what an iPod hard drive looks like in the photos above); the hard drive, in particular, was an obvious excuse for failure if you took them jogging or tossed them around in your bag. With the iPod Touch, which launched in 2007, Apple switched decisively to SSD technology, making music players thinner and lighter in your pocket, less prone to mechanical failure, and giving far better battery life. You're more likely to wear out the buttons or crack the screen on a modern iPod or iPhone than do any damage to the memory chips inside.

Here's a very quick comparison between traditional hard-disk drives (HDDs) and SSDs on a few key measurements:

Access time (ms) 10 0.1
Read speed (MB/s) 50 - 100 200 - 500
Weight (g) 500 50
Power consumption (W) 6 2 - 3

No contest? SSDs win hands down? Not so fast! If you're looking to buy as much storage as you can for as little cash, and you're less fussy about things like power consumption and speed, traditional hard drives are still the best value for money. As of 2024, SSDs are still quite a bit more expensive per gigabyte than traditional hard drives, though there are big variations in price among the different types of SSDs and the difference between SSDs and hard drives is closing year by year. As an example, in June 2024 the typical price of a Seagate 4TB hard drive on Amazon is US$70 (in the UK, £91), where a comparable 4TB Sandisk SSD drive comes in at $300 (in the UK, it's £260)—so you'll pay several times more for SSD performance. Don't expect old-style hard drives to disappear until that price difference closes substantially!

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A hard drive actuator

Photo: A hard drive actuator: it's a voice coil (or sometimes a stepper motor) that sits in the corner and swings the read-write head back and forth across the platters.



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