desktop computer and
server in use
today contains one or more hard-disk drives. Every
supercomputer is normally connected to hundreds of them. You can even
find VCR-type devices and
that use hard disks instead of
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
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
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
Let's look at the big differences between cassette tapes and hard disks:
- 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
- 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.
Because of 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 second.
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
at a time.
There are two ways to measure the performance of a hard disk:
- 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.
The other important parameter is the capacity of the drive, which
is the number of bytes it can hold.
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
In this picture you can see:
- 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!
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 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,
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
system level, sectors are often grouped together into clusters.
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!