Computer Hard Disk Drive & Major Components of a HDD

Computer Hard Disk Drive & Major Components of a HDD
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Computer Hard Disk Drive & Major Components of a Hard Disk Drive 

Computer Hard Disk Drive
Computer Hard Disk Drive

What is Hard Disk drive?

Hard disks (hard drives) are used to store large amounts of data in PCs, laptops and other digital systems. The main hard disk in a PC system is where the operating system, executable programs and files are stored.

A Hard Disk Drive (often shortened as hard disk, hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating rigid (i.e. hard) platters with magnetic surfaces. Strictly speaking, “drive” refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.

How Hard Disk Drive works?

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.

Hard Disk Drive Physical Components


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 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.

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.

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

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.

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.

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.

Sectors are often grouped together to form Clusters.

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.

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 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.

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.

Hard Disk Drive is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surface.

Types of HDD :

  1. IDE : Integrated Drive Electronics. IDE drives are also known as PATA drives( Parallel advance technology attachment )
  2. SATA : Serial advance technology attachment
  3. SCSI : Small Computer System Interface. SCSI is pronounced as scuzzy.
  4. SAS : Serial Attached SCSI

IDE / PATA (Integrated Drive Electronics Drive / Parallel Advance Technology Attachment Drive)

  • IDE/PATA Drives have usually 40 pins.
  • IDE/PATA Drives offer 133 MB/sec transfer rate.
  • It sends 8 bit data at a time.
  • PATA Cables are used to connect PATA HDD. Two drives can be connected in a single pata cable. One as master and other as slave. The configuration of master and slave is done by different combination of jumpers in the hdd.

SATA (Serial Advance Technology Attachment Drive)

  • SATA Drives have usually 7 pins, 4 pins in pair of two for sending and receiving data and rest 3 pins are grounded.
  • SATA Drives offers generally 300MB/sec transfer rate.
  • It sends data bit by bit.
  • SATA Cables are used to connect SATA HDD. Only one drive can be connected in a single sata cable.

SCSI (Small Computer System Interface Drive)

  • SCSI Drives have usually 50 to 68 pins.
  • SCSI Drive offers generally 640MB/sec transfer rate.
  • This drives are hot swappable.
  • SCSI cables are used to connect SCSI HDD. Maximum of 16 drives can be connected in a single scsi cable. Each hdd have a 8 bytes hexadecimal code known as WWN (worldwide name) for its identification in the cable.

SAS(Serial Attached SCSI Drive)

  • SAS Drives generally offers 805 MB/sec transfer rate.
  • This drives are hot swappable.
  • SAS Cables are used to connect SAS Drives. Maximum of 128 drives can be connected in a single sas cable.

HDD and SSD Comparison

The standard hard disk drive (HDD) has been the predominant storage device for desktop computers and laptops for a long time. But now, with the invention of solid state drive (SSD) technology, computer buyers and users now have access to the latest innovation that’s setting new trends in the storage market. So which one should you go for – the HDD or SSD? Read on to find out.

What is an HDD?

A hard disk drive (HDD) is basically a storage device in a computer. It is comprised of metal platters with magnetic coating, spindle, and various moving parts to process and store data. The common size for laptop hard drives in the 2.5” model, while a larger 3.5” model is usually found in desktop computers.

What is an SSD?

A solid state drive (SSD) is also another type of data storage that performs the same job as an HDD. But insteading of storing data in a magnetic coating on top of platters, an SSD uses flash memory chips and an embedded processor to store, retrieve, and cache data. It is roughly about the same size as a typical HDD, and bears the resemblance of what smartphone batteries would look like.


This is where SSDs truly prevail. While HDDs need a long time to access data and files because the disk must spin to find it, SSDs are up to 100 times faster since data can be accessed instantly. This is why an SSD-equipped PC will boot within seconds and deliver blazing fast speed for launching programs and applications, whereas a computer that uses a HDD will take much longer time to boot the operating system, and will continue to perform slower than an SSD during normal use.


As of writing, SSD units top out at 16TB storage capacity. Although there are large SSDs, anything that’s over 512GB is beyond most people’s price range. HDDs, on the other hand, have large capacities (1-2TB) available for much more affordable prices.


HDDs consist of various moving parts and components, making them susceptible to shock and damage. The longer you use your HDD, the more they wear down and most eventually end up failing. Meanwhile, an SSD uses a non-mechanical design of flash storage mounted on a circuit board, providing better performance and reliability, and making it more likely to keep your files and data safe.


An HDD can sometimes be the loudest part of your computer. Even the highest-performing HDDs will emit some noise when the drive is spinning back and forth to process data. SSDs have no moving parts, meaning it makes no noise at all.


More moving part means more heat, and HDD users will have to live with the fact that their device will degenerate over time. SSD uses flash memory, generating less heat, helping to increase its lifespan.


To be frank, SSDs are much more expensive than HDDs for the same capacity. This is why most computers with an SSD only have a few hundred gigabytes of storage. HDDs are about twice as cheaper than SSDs.

Despite the high costs and low capacity, SSD is a clear winner over the HDD in terms of performance. While you’re paying more for less memory with an SSD, you’re investing in a faster and far more durable data storage option in the long run.

HDD and SSD Comparison
HDD and SSD Comparison

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