Understanding Hard Drive
Terminology
Disk geometry
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The following sections introduce you to disk geometry —
essentially the physical components of a drive that make up
your data storage solution.You also find out some general
terminology about hard drives and hard drive storage in this
section.
Platters
 A platter is a physical object (actually, a plate) inside the hard disk
that is responsible for storing the data.
 A platter is similar to a music record and a hard disk has many
platters.
 The platters are similar to records on a record player in the
sense that they spin on a spindle that runs through the center
of all the platters.
Disk geometry
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Each platter has two sides for storing information, and each side of
the platter has a unique ID.
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The ID for the first side of the first platter is 0, and each side
increases by 1.
For example, two platters are in the disk, the first platter has
side 0 and side 1, and the second platter has side 2 and side 3.
Because each side of the platter has a writing mechanism, many
people use the terms “head” and “side” interchangeably.
The head is more accurately called the read/write head because
it moves over the disk surface and reads from or writes to the
disk.
Like a needle on a record player, the read/write head moves
over the surface of the disk with the help of an arm, called the
actuator arm or the head positioning mechanism.
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Disk geometry
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Each platter surface on the disk has its own read/write head.
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When information is written to the disk, the read/write
head moves to the same track on all platters in a single
movement and then writes the data across the same
track on all platters.
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The actuator arm has multiple read/write heads on it.
Tracks
 Just like there are grooves (tracks) on a music record,
there are also tracks on each platter.
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These tracks are evenly spaced across the platter’s surface.
Disk geometry
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Sectors
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The platter is divided into pie-shaped slices, called sectors.
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Now the confusing thing about sectors is that where a
track intersects with a sector, sector blocks are created —
also known as sectors!
Each sector (block) — 512 bytes in size — is the actual
storage area for data.
Each pie-sliced sector has an address; the first sector is
sector 1, the second sector is sector 2, and so on.
So, each sector block has an address comprising the
platter side number, the sector, and track number.
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Disk geometry
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Note that the term sector block is a term that I made up for
this discussion; the term sector is also used to describe the
512-byte blocks.
Clusters
 A group of sectors makes up a cluster, which is the
allocation unit for a file — meaning where a file is saved.
 When a partition is formatted, the file system determines
the cluster size based upon the partition size.
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For example, data can be saved to side 1, sector 2, track 4 —
which is the address of a 512-byte sector block.
Disk geometry
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For example, a 2GB FAT partition uses a 32K cluster size.
That same 2GB partition formatted as FAT32 uses only a 4K
cluster size.
Having a partition use a 4K cluster size means that eight
sectors make up a cluster.
Keep in mind that after a file is saved to the cluster, no other
file can occupy that cluster.
For example, if you have a 32K cluster size and you save a
3K file to the hard disk, the file is saved to an empty cluster
— but only 3K of that cluster is used, and the remaining 29K
is empty.
The remaining 29K is now considered unusable space; no
other file can be saved to that unused 29K.
Disk geometry
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Cylinders
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All platters in the hard disk contain the same number of
tracks, but that number varies from one hard disk to
another.
These tracks are numbered from the outside in, starting
with 0 (zero).
For example, on a platter with ten tracks, the track
closest to the outer edge of the platter is track 0, and the
track closest to the center is track 9.
A cylinder consists of the same track on both sides of
all the platters.
Disk geometry
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In other words, when you reference track 0, you
reference a particular track on a particular platter;
however, when you reference cylinder 0, you reference
track 0 on all platters.
If you know the number of cylinders, heads, and sectors
per track, you can calculate the size of a disk.
For example, if a drive has 4,092 cylinders, 16 heads, and
63 sectors per track, the size of the disk is 2,111,864,832
bytes (2.1GB).
The formula to calculate the size of the disk is cylinders ×
number of heads × number of sectors per track × 512
bytes per sector
Read/write process
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Platters are divided into 512-byte sectors.
These sectors are the area on the platter that data is
written to.
The platters have a magnetic coating applied that is
extremely sensitive to magnetism.
While the platters spin, the read/write head moves
from track to track until it reaches the desired track.
Then it waits for the appropriate sector to move
underneath it, at which time the read/write head is
energized to apply a magnetic charge to the particles
in the disk coating.
Read/write process
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This changes the particle binary state from 0 to 1, thus
creating data.
The same happens when the data needs to be read: The
read/write head moves over the appropriate sector and reads
the data that resides in the sector.
The read/write heads don’t actually touch the surface of the
disk platters; instead, they hover about 10 micro-inches (or
millionths of an inch) above it. (That’s not even enough space
to place a hair between the read/write head and the platter’s
surface.)
This design helps improve disk performance because a
read/write head that makes contact with the platter causes
friction, slowing down the rotation speed of the disk and
creating extra heat.
Performance
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Disk performance can be measured in terms of several
important characteristics:
Seek time is how long it takes to move the read/write heads
to the desired track. Seek time is measured in milliseconds (ms),
or one-thousandth of a second.
Latency is how long it takes for the appropriate sector to
move under the read/write head. Latency is measured in
milliseconds.
Access time describes the overall speed of the disk. It is a
combination of seek time and latency. The lower the access time,
the better.
Spin speed is how fast the platters spin, measured in rotations
per minute (rpm). The larger the rpm value, the faster the disk,
which means less latency.
Master Boot Record
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The Master Boot Record (MBR) is the first sector on the first track of
the first side of the first platter; it holds the operating system
(OS) boot code that controls the loading of the OS.
The MBR also holds drive characteristics, such as the partition
table. During the boot process, the system has to find a primary
partition that is active — it does this by looking at the partition
table in the MBR.
In general, if anything goes wrong with the MBR, you will not be
able to boot the system. Because the boot record is always in
the same location on every disk, it becomes very easy for a
malicious hacker to write viruses that modify or corrupt the
MBR. This is one reason you should always run virus-detection
software.
LBA and ECHS
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Logical Block Addressing (LBA) and Extended Cylinder/Head/Sector
(ECHS)
Essentially, LBA and ECHS perform the same goal: namely,
performing sector translation, which is the hard drive controller
lying to the BIOS about the drive geometry.
LBA was developed by Western Digital, and ECHS was Seagate’s
solution to recognizing larger drives.
You need sector translation because the original BIOS code
found on computers was limited to seeing only 1024 cylinders,
16 heads, and 63 sectors — which is a total drive size of 504MB
(1024 × 16 × 63 × 512).
However, if you bought a 2.1GB hard disk, your BIOS would not
recognize it because the geometry of the 2.1GB drive is too high
for the BIOS.
LBA and ECHS
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In this example, the geometry of the drive is 16,384 clusters, 4
heads, and 63 sectors.
LBA and ECHS
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Here’s an example of why you take the lowest value in each
category. If the hard disk supports only 4 heads, only 4 heads are
detected. Although the BIOS supports a potential 16 heads, that
doesn’t mean that they are actually there.
So the problem is that you purchased a 2.1GB drive, but the
system recognizes only 132MB! The solution to this problem is
LBA or ECHS — again, both technologies offer the same
solution. They were just built by different manufacturers.
An LBA-enabled BIOS can recognize 1024 cylinders, 256 heads,
and 63 sectors — essentially being able to support more heads
on the drive. As a result, the drive lies to the BIOS by using a
translation factor of usually 2, 4, 8, or 16. The physical dimensions
of the drive are taken and manipulated by the translation factor
to calculate the logical dimensions reported to the BIOS.
LBA and ECHS
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In my example, 16,384 cylinders are too many cylinders, so they
are divided by translation factor of 16 to reach the LBA
maximum number of cylinders supported. To make up for the
loss in cylinders, the heads are then multiplied by 16, ensuring
that the logical number of heads falls under the LBA limit of 254.
LBA and ECHS
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To leverage larger size drives, your BIOS would have to support
LBA or ECHS — which most BIOS do today.
Notice that an LBA-enabled BIOS can support only an 8.4GB
drive — and we are way past that drive size today.
Today’s BIOS support the INT13 extensions, developed by
Phoenix Technologies, which allow the systems to see drives past
137GB in size!
The BIOS can recognize larger size drives because it simply
identifies the drives by the number of sectors.
Basic Hard Disk Drive Components
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The basic components of a typical hard disk drive are as follows
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■ Disk platters
■ Read/write heads
■ Head actuator mechanism
■ Spindle motor (inside platter hub)
■ Logic board (controller or Printed Circuit Board)
■ Cables and connectors
■ Configuration items (such as jumpers or switches)
The platters, spindle motor, heads, and head actuator
mechanisms usually are contained in a sealed chamber called the
head disk assembly (HDA).
Other parts external to the drive’s HDA, such as the logic
boards, bezel, and other configuration or mounting hardware,
can be disassembled from the drive.
Basic Hard Disk Drive Components
Hard Disk Platters (Disks)
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A hard disk drive has one or more platters, or disks. Hard disks
for PC systems have been available in several form factors over
the years. Normally, the physical size of a drive is expressed as
the size of the platters.
Platters were originally made from an aluminum/magnesium alloy,
which provides both strength and light weight.
However, manufacturers’ desire for higher and higher densities
and smaller drives has led to the use of platters made of glass
(or, more technically, a glass-ceramic composite).
One such material, produced by the Dow Corning Corporation,
is called MemCor.
MemCor is composed of glass with ceramic implants, enabling it
to resist cracking better than pure glass.
Recording Media
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No matter which substrate is used, the platters are covered with
a thin layer of a magnetically retentive substance, called the
medium, on which magnetic information is stored.Three popular types
of magnetic media are used on hard disk platters:
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■ Oxide media
■ Thin-film media
■ AFC (antiferromagnetically coupled) media
IBM introduced AFC media starting with the 2 1/2" Travelstar
30GN series of notebook drives introduced in 2001; they were
the first drives on the market to use AFC media.
Thin-film sputtered media are created by first coating the
aluminum platters with a layer of nickel phosphorus and then
applying the cobalt-alloy magnetic material in a continuous
vacuum-deposition process called sputtering.
Read/Write Heads
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A hard disk drive usually has one read/write head for each
platter surface (meaning that each platter has two sets of
read/write heads—one for the top side and one for the bottom
side).
These heads are connected, or ganged, on a single movement
mechanism.The heads, therefore, move across the platters in unison.
As disk drive technology has evolved, so has the design of the
read/write head. The earliest heads were simple iron cores with
coil windings (electromagnets). By today’s standards, the original
head designs were enormous in physical size and operated at
very low recording densities. Over the years, head designs have
evolved from the first simple ferrite core designs into the
magneto-resistive and giant magneto-resistive types available
today.
Head Actuator Mechanisms
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Possibly more important than the heads themselves is the
mechanical system that moves them: the head actuator.
This mechanism moves the heads across the disk and positions
them accurately above the desired cylinder. Many variations on
head actuator mechanisms are in use, but all fall into one of two
basic categories:
■ Stepper motor actuators
■ Voice coil actuators
The use of one or the other type of actuator has profound
effects on a drive’s performance and reliability. The effects are
not limited to speed; they also include accuracy, sensitivity to
temperature, position, vibration, and overall reliability.
Head Actuator Mechanisms
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Stepper Motor Actuators
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A stepper motor is an electrical motor that can “step,” or move,
from position to position, with mechanical detents or click-stop
positions. If you were to grip the spindle of one of these motors
and spin it manually, you would hear a clicking or buzzing sound
as the motor passed each detent position with a soft click.
Voice Coil Actuators
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The voice coil actuators used in virtually all hard disk drives
made today—unlike stepper motor actuators—use a feedback
signal from the drive to accurately determine the head positions
and adjust them, if necessary. This arrangement provides
significantly greater performance, accuracy, and reliability than
traditional stepper motor actuator designs.
Head Actuator Mechanisms
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The two main types of voice-coil positioner mechanisms
are
■ Linear voice-coil actuators
■ Rotary voice-coil actuators
The two types differ only in the physical arrangement of
the magnets and coils.
Servo Mechanisms
Three servo mechanism designs have been used to control
voice coil positioners over the years:
■ Wedge servo
■ Embedded servo
■ Dedicated servo
Air Filters
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Nearly all hard disk drives have two air filters.
One is called the recirculating filter, and the other is called
either a barometric or breather filter.
These filters are permanently sealed inside the drive and
are designed never to be changed for the life of the drive,
unlike many older mainframe hard disks that had
changeable filters.
Hard Disk Temperature Acclimation
Because most hard drives have a filtered port to bleed air
in to or out of the HDA, moisture can enter the drive, and
after some period of time, it must be assumed that the
humidity inside any hard disk is similar to that outside the
drive.
Spindle Motors
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The motor that spins the platters is called the spindle motor
because it is connected to the spindle around which the platters
revolve.
Spindle motors in hard disk drives are always connected directly;
no belts or gears are involved.
The motor must be free of noise and vibration; otherwise, it can
transmit a rumble to the platters, which can disrupt reading and
writing operations.
The spindle motor also must be precisely controlled for speed.
The platters in hard disk drives revolve at speeds ranging from
3,600 rpm to 15,000 rpm (60–250 revolutions per second) or
more, and the motor has a control circuit with a feedback loop
to monitor and control this speed precisely.
Other parts
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All hard disk drives have one or more logic boards mounted on
them. The logic boards contain the electronics that control the
drive’s spindle and head actuator systems and present data to
the controller in some agreed-upon form.
Hard disk drives typically have several connectors for interfacing
to the computer, receiving power, and sometimes grounding to
the system chassis. Most drives have at least these three types of
connectors:
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■ Interface connector(s)
■ Power connector
■ Optional ground connector (tab)
To configure a hard disk drive for installation in a system, you
usually must set several jumpers (and, possibly, terminating
resistors) properly.