Hard drives
Last updated: May 24, 2007.
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
is 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. 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.
Magnetic memories
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 using 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.
Caption: Magnets and paper clips—the technology behind
hard drives
really is this simple!
This artwork comes from NASA, with thanks.
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 microsopically 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.
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.
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 and move radially
(in a straight line 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.
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.
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.
