by Chris Woodford. Last updated: April 18, 2014.
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. An iPod
(one of the older ones, anyway) is 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. 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 typical 30GB (gigabyte) hard drive from a 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.
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.
Photo: Magnets—the technology behind hard drives really is this simple!
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 1.6 million
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
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...
- 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.
- Read-write arm swings read-write head back and forth across
- Central spindle allows platter to rotate at high speed.
- Magnetic platter stores information in binary form.
- Plug connections link hard drive to circuit board in personal
- Read-write head is a tiny magnet on the end of the read-write arm.
- Circuit board on underside controls the flow of data to and from
- Flexible connector carries data from circuit board to read-write
head and platter.
- Small spindle allows read-write arm to swing across platter.
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 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
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.
track is broken up into smaller areas called sectors.
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.
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. Left: The actuator arm swings the head back and
forth so it's in the right position on the drive. Right: 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 right photo is a
reflection in the shiny hard drive surface!
Find out more
On this website
You might like these other articles on our site covering related topics:
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.
- Hard Disk Drive: Mechatronics and Control by Abdullah Al Mamun, Guoxiao Guo, Chao Bi. CRC Press, 2007. A detailed reference covering the design and manufacture of hard drives.
- Upgrading and Repairing PCs by Scott Mueller. Que Publishing, 2003. Chapter 10 "Hard Disk Storage" (p.601) is a really good, clear introduction.
- Hard Drive Bible by Martin Bodo. Corporate Systems Center, 1996. A slightly dated but still very useful reference.
- Hard Drive: As the Disk Turns by Gordon Hughes. Createspace Publishing (Amazon), 2007. A less technical read than the reference books up above. This one's the story of Seagate and how it became a major force in hard-drive manufacturing.