You all know about Hard Disk and 95% of us used Hard Disk in our P.C. But it’s a big fact the lower than 50% of us don’t know about the structure of Hard Disk. Now I going to reveled the internal structure of HDD and how its work.
A hard disk is a sealed unit containing a number of
platters in a stack. Hard disks may be mounted in a horizontal or a vertical
position. In this description, the hard drive is mounted horizontally.
Electromagnetic read/write heads are positioned above
and below each platter. As the platters spin, the drive heads move in toward
the center surface and out toward the edge. In this way, the drive heads can
reach the entire surface of each platter.
Basically structure of Hard Disk in Two
types
1. Physical Structure
2. Logical Structure
Physical Structure
Hard drive
physical components
PLATTERS:
Platter
is a circular, metal disk that is mounted inside a hard disk drive. Several
platters are mounted on a fixed spindle motor to create more data storage
surfaces in a smaller area. The platter has a core made up of aluminium or glass substrate, covered with a thin layer of
Ferric oxide or cobalt alloy. On both sides of the substrate material, a thin
coating is deposited by a special manufacturing technique. This, thin coating
where actual data is stored is the media layer.
When
the magnetic media is applied to the surface of the substrate material, a thin
lubricating layer is applied to protect the material. This complex three
layered media is discussed in detail as follows:
·
THE
SUBSTRATE MATERIAL:
The
bulk material of which platters are made up, forms the base on which media
layer is deposited. The substrate has no specific
function but to support the media layer. The most commonly used material for
making this physical layer is an Aluminium alloy.
This alloy is rigid, lightweight, stable, inexpensive, easy
to work with and is readily available. Earlier, since the gap between the heads
and the platter was relatively high, the platter surface being smooth and flat
was less of an issue. However, as technology advances, the gap between heads
and platters is decreasing and the speed that the platters spin at is
increasing. For this reason demand for alternatives on the platter material are
increasing. Glass platters are replacing aluminium
platters because they provide improved rigidity, better quality, thinner
platters, and thermal stability.
·
MEDIA
LAYER:
The
substrate material forms the base upon which actual recording media is
deposited. The media layer is a thin coating of magnetic material applied to
the surface of the platters and where the actual data is stored. Its thickness
is only a few millionths of an inch.
Special
techniques are employed for the deposition of magnetic material on the
substrate material. A thin coating is deposited on both sides of the substrate,
mostly by vacuum deposition process called magnetron sputtering. Another such
method is electroplating, using a process similar to that used in
electroplating jewelry.
·
PROTECTIVE
LAYER:
On
the top of the magnetic media, is applied a super-thin, protective, lubricating
layer. This layer is called the protective layer because it protects the disk
from damage caused by accidental contact from the heads, “head crash” or other
foreign material from entering the drive
·
PLATTER
DIVISIONS:
In
order to get maintain the organized storage and retrieval of data the platters
are organized into specific structures. These specific structures include
tracks, sectors, and clusters.
1)
TRACKS:
Each
platter is broken into thousands of tightly packed concentric circles, known as
tracks. These tracks resemble the structure of annual rings of a tree. All the
information stored on the hard disk is recorded in tracks. Starting from zero
at the outer side of the platter, the number of tracks goes on increasing to
the inner side. Each track can hold a large amount of data counting to
thousands of bytes.
2)
SECTORS:
Each
track is further broken down into smaller units called sectors. As sector is
the basic unit of data storage on a hard disk. A single track typically can
have thousands of sectors and each sector can hold more than 512 bytes of data.
A few additional bytes are required for control structures and error detection
and correction.
3)
CLUSTERS:
Each
partition on your hard disk is subdivided into clusters. A cluster is the
smallest possible unit of storage on a hard disk. The size of a cluster depends
on two things:
·
The size of the partition
·
The file system installed on the
partition
Sectors
are often grouped together to form Clusters.
READ/WRITE HEADS:
The
heads are an interface between the magnetic media where the data is stored and
electronic components in the hard disk. The heads convert the information,
which is in the form of bits to magnetic pulses when it is to be stored on the
platter and reverses the process while reading. The heads are the most
sophisticated part of the hard disk. Each platter has two read/write heads, one
mounted on the top and the other one at the bottom. These heads are mounted on
head sliders, which are suspended at the ends of head arms. The head arms are
all fused into a singular structure called actuator, which is responsible for
their movement.
THE SPINDLE MOTOR:
Spindle
motor plays an important role in hard drive operation by turning the hard disk
platters. A spindle motor must provide stable, reliable, and consistent turning
power for many hours of continuous use. Many hard drive failures occur due to
spindle motor not functioning properly
HARD DISK LOGIC BOARD:
Hard disk is made with an intelligent circuit board integrated into the hard disk unit. It is mounted on the bottom of the base casting exposed to the outer side. The read/write heads are linked to the logic board through a flexible ribbon cable.
DRIVE BAY:
The entire
hard disk is mounted in an enclosure designed to protect it from the outside
air. It is necessary to keep the internal environment of the hard disk free of
dust and other contaminants. These contaminants may get accumulated in the gap
between the read/write heads and the platters, which usually leads to head
crashes. The bottom of the disk is also called base casting. The drive
mechanics are placed in the base casting and a cover, usually made up of aluminium is placed on top to enclose heads and platters.
The entire contents placed on the base and cover chamber are collectively known
as the head-disk assembly. Once this assembly is opened, it would instantly
contaminate the contents and eventually ruin the drive.
On the bottom of the base casting is present the logic board, which is separated from the base casting using a cushioning material.
Logical
Structure
A hard disk
drive has a logical structure that is compatible with the operating system
installed. The Master Boot Record is the most important part of the hard disk.
It is the first sector on the hard disk that the operating system accesses. The
Master Boot Record contains the Boot Loader and Partition Tables. It is
important to know about the logical structure of the hard disk to gain better
understanding and fix issues related to the hard disk drive.
Master Boot Record (or MBR)
·
At the beginning of the hard drive is the MBR.
When your computer starts using your hard drive, this is where it looks first.
·
The MBR itself has a specific organization. The
size of the MBR is 512 bytes.
·
The boot loader is the first 446 bytes of the
MBR. This section contains executable code, where programs are housed.
· The partition tables are 4 slots of 16 bytes each, containing the description of a partition (primary or extended) on the disk.
Here
is how to describe a partition:
1.
State of the partition (inactive partition
bootable) - (1 byte)
2.
Custom heads at the beginning of the partition -
(1 byte)
3.
Cylinder sector and the beginning of the
partition - (2 bytes)
4.
Type of partition (file system, eg, 32 fat, etc ...) - (1 bytes)
5.
Head of the end of the partition (1 byte)
6.
Cylinder sector and the end of the score - (2
bytes)
7.
Number of sectors between the MBR and the first
sector of the partition - (4 bytes)
8. Number of sector of the partition - (4 bytes)
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