Understanding Hard Drive Terminology
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Understanding Hard Drive Terminology

Disk geometry. 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..

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Understanding Hard Drive Terminology

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Understanding Hard Drive Terminology


Disk geometry

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.



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

Each platter has two sides for storing information, and each side of the platter has a unique ID.

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.


Disk geometry

Each platter surface on the disk has its own read/write head.

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.

The actuator arm has multiple read/write heads on it.


Just like there are grooves (tracks) on a music record, there are also


on each platter.

These tracks are evenly spaced across the platter’s surface.



Disk geometry


The platter is divided into pie-shaped slices, called


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.


Disk geometry

For example, data can be saved to side 1, sector 2, track 4 — which is the address of a 512-byte sector block.

Note that the term

sector block is a term that I made up for this discussion; the


sector is also used to describe the 512-byte blocks.


A group of sectors makes up a

cluster, which is the allocation unit for a file —

meaning where a file is saved.

When a


is formatted, the file system determines the cluster size based upon the

partition size.


Disk geometry

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


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.


cylinder consists of the same track on both sides of all the platters.


Disk geometry

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


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

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

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.



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.


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


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.



Logical Block Addressing (LBA)


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.



In this example, the geometry of the drive is 16,384 clusters, 4 heads, and 63 sectors.



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.



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.



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

The basic

components of a typical hard disk drive are as


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



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)

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


/magnesium alloy, which provides both strength

and light



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



such material

, produced by the Dow Corning Corporation, is called




is composed of

glass with

ceramic implants, enabling it to resist cracking better than pure glass.


Recording Media

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



media are used on hard disk platters


■ Oxide media

■ Thin-film media

■ AFC (




IBM introduced AFC media starting with the 2 1/2"


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


platters with a layer of

nickel phosphorus

and then applying the cobalt-alloy magnetic material in a continuous

vacuum-deposition process




Read/Write Heads

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





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

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


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

Stepper Motor


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


The voice coil actuators used in virtually all hard disk drives made today—unlike stepper motor


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

The two main types of voice-coil




■ 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


over the years:

■ Wedge servo

■ Embedded servo

■ Dedicated servo


Air Filters

Nearly all hard disk drives have two air filters


One is called the


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


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

The motor that spins the platters is called the

spindle motor because it is connected to the



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.


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

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.


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


■ 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.