by Chris Woodford. Last updated: June 1, 2019.
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
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 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.
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.
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.
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
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.
Photo: The read-write head on a hard-drive. 1) The actuator arm swings the head back and
forth so it's in the right position on the drive. 2) 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 the second photo is a
reflection in the shiny hard drive surface!
Who invented the hard drive?
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.
Artwork: The original hard drive. 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). 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.
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.
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
|Access time (ms)
|Read speed (MB/s)
||50 - 100
||200 - 500
|Power consumption (W)
||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 2019, SSDs are still typically several times more expensive per gigabyte than traditional hard drives. Don't expect old-style hard drives to disappear until that price difference closes substantially!