How Computer Hard Disk Drive works

what is Hard disk drive

Before  we look into how Computer Hard Disk Drive works, we shall start by defining what a hdd is. A hard disk drive is an electro-mechanically controlled non-volatile data storage device. It is used for data storage and retrieval. Your data is written or erased on a platter that is coated with magnetic material.

history of computer drive

internal components of a computer hard drive

Today, more than 70% of the computers in the market uses HDD as their primary storage device. However, today some computers are using SSD ( Solid-state drive) storage devices. In case you are wondering what is a solid-state drive (SSD), they are NAND flash memory. In other words, they are like RAM only that they store data even when not connected to power. SSD are better storage devices that have emerged today that offer higher reliability, speed, and performance. Other names for hard disk drives are hard drive and fixed drive.

Hard disk form factor

You may have seen how big a desktop computer HDD is. You may have also seen that those used in the laptops are smaller in size.  These are the two form factors for hard disk drives. That form factor used in the desktops is 3.5 inches while that use in the laptops is 2.5 inches. Other than form factors, HDD comes with various speeds. The lowest speed in a hard drive is 1200 rpm while it can go as high as 15000 rpm.  However, you might be having one that has RPM rates between 5400 and 7200 RPM.

High speeds do not always translate to high performance.  The higher the speed, the higher the temperatures emitted. High temperatures introduce thermal throttling in your pc performance.

History of HDD

The first hard drive was developed and introduced into the market by IMB in the year 1956. The HDD capacity was only 5MBs and was priced at $ 50,000.  Unlike today’s HDD you and I know, that one could not be removed from its computer; RAMAC 305 system.  

The same company made another HDD, now removed one but smaller in size-2.6Mb. it was not until 1980 that one gigabyte HDD was developed and introduced into the market. The cost of 1Gb was $ 40,000 and weighed more than 200 kg.

Seagate introduced its first HDD drive that had a speed of 7200 RPM in the year 1992. By the end of 1996, another HDD from Seagate enter the market with 10000 RPM.Now that we know some of the hdd history, lets take a look at How Computer Hard Disk Drive works


The components of an HDD include the 2 motors, spindle, platter, actuator arm actuator, power connectors, jumper block, disk controller, and IDE connectors. Every hard disk drive has two motors; one to rotate the disk and the other to control the actuator. The disk controller is located behind the platter. The disk controller is a circuit board that allows the computer and hard drive to communicate with one another.

SATA drives

SATA is an acronym for serial advanced technology attachment.  This is the go-to interface for most HDD used in desktops and laptops. Actually, the name describes how the hard disk drive is connected to the motherboard. SATA interface offers faster data retrieval than the PATA hard drive. SATA is known to write HDD at a speed of 6 Gb/s with a throughput of 600Mb/s.

Hard drives come in all possible sizes.  The least size available in the market today is 250 Gb. You can find the following capacity available for enterprise, 250GB, 320 Gb, 500Gb, 1Tb, 2TB, 4Tb, and even 16 Tb.

Compared to other storage media, the hard drive is the most affordable one you can get in the market. However, they are limited to uses that are not extremely high read and writes.

However, despite their low cost, you may realize they are prone to shocks and sudden movements. In other words, they may fail if you dropped them from a height. This should not stop you from going for one. They have a considerably long life span but will soon be phased out of the market if the cost of SSD devices drops.

Other than SATA, there are methods of connecting the HDD to the motherboard. The two other methods are IDE and SCSI. It is important for you to know that HDD required one to connect to a power supply separately.  The amount of power required to run an HDD is enormous. This is why the more HDD you have in your PC, the more power you will consume. This, however, should not prevent you from installing as many HDD as your motherboard allows.

How is data stored and read in HDD?

When you are saving something in your HDD, the information is sent to the disk controller first. Your disk controller instructs the HDD how to move components around. When the OS requires to read or write data, it first checks the HDD`s file allocation table (FAT).

History of computer hard drive

reading and writing process of a hard disk drive

The FAT tells the OS where the required data is located as well as areas available for writing. When the area is located, the disk controller instructs the actuator to move the arm and align the read-write head. The data is usually scattered to so many places and so the head needs to move from one location to another while reading or writing data.

The information is stored by magnetizing the platter as shown in the figure below;

When reading the data, the read/write head reads the magnetic polarities on the plater.


You now know that the data is read and written on HDD by a moving head. However, SSD does not have any moving parts. This is the major physical difference between the two. SSD attributes include high reliability and speed that makes them the best choice for critical data that needs to be accessed quickly.

In terms of price, SSD storage devices are way more expensive than HDD of equal storage capacity.  To reduce the cost while increasing performance, many companies always go for hybrid storage. Additionally, SSD storage has a lower latency than HDD.

Internal and external HDD

HDD can further be categorized into both internal and external. An internal hard drive is one located inside your computer and you rarely remove it. The external hard drive is a standalone drive that you use as a flash drive to store data and also back up data. External hard drive either comes in enclosures or have to be put in enclosures.  The encloses have interfaces that allow you HDD to connect to your computer.  You can connect an external HDD to a computer using USB,  eSATA, or firewire.

Mostly 2.5 inches HDD form factors are preferable external HDD because they can sufficiently be powered by a single USB cable.  If the 3.5 inches form factor is to be used, you will need both a USB cable and an additional power adaptor.  The USB is for data transfer while the power adapter is for powering your HDD. Now you know how  Computer Hard Disk Drive works