Hard disk drives or HDDs is a type of data storage. Its technology is based on electromagnetism. It is designed to store data. A hard disk drive is non volatile and retains data even while turned off. Data is accessed in a random access manner meaning that data can be retrieved non sequentially in any order.


Hard disk drives were invented by IBM in 1954. The work was done by a team led by Rey Johnson and the HDDs hit the market in 1956. They immediately became the dominant storage option for computers until SSDs showed up.

The major manufacturers for Hard disk drives are Seagate, Toshiba and Western Digital.

Key specs

Form factor: There two main form factors for HDDs are 3.5″ (desktops) and 2.5″ for laptops.

Storage capacity

Connections: PATA, SATA, USB or SCSI


Speed: HDD speed is measured in RPMs or Revolutions Per Minute. Faster is always better.

Read: Smartphone storage types: an overview

Parts of a Hard Disk Drive

  1. Actuator: They move the read-write arm.
  2. Actuator Read-Write Arm: moves the read-write heads
  3. Read-Write head: A tiny magnet that reads and writes data
  4. Platter: A magnetic plate that stores information
  5. Control spindle: rotates the platter
  6. Actuator spindle: rotates the platter
  7. Connection to PC
  8. Circuit board with controller for data control (read, write and access)
  9. Flexible connector: moves data to and fro from circuit board to plaster and back

How HDDs work

Hard disk drives store data using magnetism. The data is stored on the platter, divided into billions of tiny areas. Each one of these areas can be magnetized (1) or demagnetized (0) to store data.

Reading-Writing data

There are 2 read-write heads per platter, one to read and the other to write data. To read, the actuator moves the actuator arm over the platter so that the read head can read the data stored on it.

To write data, the actuator arm moves the write head to empty sections of the platter where it can magnetize areas into 1s and 0s (110010011). This action is how data is saved (written) to a Hard Drive.

In order for a computer to locate files or write to a HDDs, a full file map of the HDD is saved to the circuit board. This file map is called a File Allocation Table (FAT) on Windows. This file map is manage by a controller.

The controller on the motherboard serves as an interface that allows computers to access, read and modify data on a HDD.

Benefits of using a HDD

  1. Affordable/cost effective
  2. High capacity storage
  3. Long life span


  1. Disk crash
  2. High power consumption
  3. Bulky
  4. Slow
  5. Reliability

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