We take it all for granted. Keeping all your memories on a little device called the mobile phone, having a plethora of books on your e-reader, and what-not. But behind all the magic is a wonder of material science, quantum physics, and magnetism. What is the working principle behind hard disks, what is it made of and how has it evolved over time?!

Image result for hard disk internal structure

Working Principle:

Hard disks are a collection of circular platters mounted on a cylinder. These disks have a magnetic surface that stores information. The surface comprises of several ‘magnetic grains’. These grains are grouped and each group is known as a ‘bit’ or ‘binary digit’.

Image result for magnetic grains on hard disks

The grains of a bit have a particular direction. Bits with grains in one direction are taken as the 1s while the ones with the other direction are taken as the 0s. 1s and 0s collectively give information in binary. This can be rendered in human-readable form. For instance, the letter J would have a binary value of 01001010.

These circular platters are constantly rotating.  When you write on a disk, data is fed into disks by converting strings of bits onto electric current by a high power electromagnet. This electromagnet generates a field strong enough to change the direction of the magnetic grains’ magnetization. For reading, a magnetic reader converts this back to an interpretable format.


The primary method to increase disk storage capacity is to increase the number of bits that can be stored on a disk. Since the introduction of hard disks by IBM in 1957, the disks’ areal capacity has increased 300 million times! Today’s modern day disks can store about 600 gigs per square inch. Pretty neat, huh?!

This transformation has taken a massive amount of research. The reader and writer were made smaller and more sensitive, and development in material sciences and quantum physics have made metallic grains smaller.