Nearly every desktop
computer and server
in use today contains one or more hard-disk drives. Every mainframe
is normally connected to hundreds of them. You can even find
VCR-type devices and camcorders
that use hard disks instead of tape. These
billions of hard disks do one thing well -- they store
changing digital information in a relatively permanent form.
They give computers the ability to remember things when the
power goes out.
In this edition of HowStuffWorks,
we'll take apart a hard disk so that you can see what's
inside, and also discuss how they organize the gigabytes of
information they hold in files!
Hard Disk Basics
Hard disks were invented in
the 1950s. They 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 tapes and floppies.
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.
Let's look at the big differences between cassette tapes
and hard disks:
these differences, a modern hard disk is able to store an
amazing amount of information in a small space. A hard disk
can also access any of its information in a fraction of a
- The magnetic recording material on a cassette tape is
coated onto a thin plastic strip. In a hard disk, the
magnetic recording material is layered onto a high-precision
aluminum or glass disk. The hard-disk platter is then
polished to mirror-type smoothness.
- With a tape, you have to fast-forward or reverse to get
to any particular point on the tape. This can take several
minutes with a long tape. On a hard disk, you can move to
any point on the surface of the disk almost instantly.
- In a cassette-tape deck, the read/write head touches the
tape directly. In a hard disk, the read/write head "flies"
over the disk, never actually touching it.
- The tape in a cassette-tape deck moves over the head at
about 2 inches (about 5.08 cm) per second. A hard-disk
platter can spin underneath its head at speeds up to 3,000
inches per second (about 170 mph or 272 kph)!
- The information on a hard disk is stored in extremely
small magnetic domains compared to a cassette tape's. The
size of these domains is made possible by the precision of
the platter and the speed of the medium.
A typical desktop machine 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
The other important
parameter is the capacity of the drive, which is the
number of bytes it can hold.
- Data rate - The data rate is the number of bytes
per second that the drive can deliver to the CPU. Rates
between 5 and 40 megabytes per second are common.
- Seek time - The seek time is the amount of time
between when the CPU requests a file and when the first byte
of the file is sent to the CPU. Times between 10 and 20
milliseconds are common.
Inside a Hard Disk
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
platters and six read/write heads:
- The platters, which typically spin at 3,600 or
7,200 rpm when the drive is operating. These platters are
manufactured to amazing tolerances and are mirror-smooth (as
you can see in this interesting self-portrait of the
author... no easy way to avoid that!).
- The arm that holds the read/write heads is
controlled by the mechanism in the upper-left corner, and is
able to move the heads from the hub to the edge of the
drive. The arm and its movement mechanism are extremely
light and fast. The arm on a typical hard-disk drive can
move from hub to edge and back up to 50 times per second --
it is an amazing thing to watch!
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.
Storing the Data
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
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!
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