How a Hard Drive Works (Video)

Generally speaking, there are three things a user wants from a hard drive: it should be fast, it should hold everything you throw at it, and it should never fail. As an electromechanical device, a hard drive can't guarantee any of these three things 100 percent, but this could be understood much more effectively if you know how a hard drive works up close. To our ears it may be just “something that makes a bit of noise” inside the computer, but in reality it is an extraordinary piece of engineering.

There are those who believe that YouTube is only good for watching music videos, ridiculous celebrity moments, state-of-the-art devices being destroyed by a shredder, or cats doing cute things. However, the educational potential of YouTube is enormous. Besides having a huge number of tutorials and instructions, it also allows us to learn how some things work in a much more visual and dynamic way.

Usually, a hard drive is a mystery to most users. Everyone depends on them, and when one breaks it typically causes a catastrophe, represented by massive data loss. But instead of asking the deity of the moment why these things happen, it's better to know up close how a hard drive works.

How a hard drive works

A hard drive uses well-known magnetic storage. This is based on detecting patterns of magnetization on a ferromagnetic material, along with their changes, interpreting them as data in binary format. The head of a hard drive, essentially an electromagnet, can modify the magnetization on the platter (writing) or measure its polarization (reading). The level of precision required for the head is impressive, and for that it relies on the so-called “voice coil actuator”, whose name comes from the voice coils used in loudspeakers.

The base of the arm sits between two very powerful magnets (I've used them to repair and hold multiple things once extracted from broken hard drives). The movement of the arm itself arises from the Lorentz force. By passing a current through a wire in an electromagnetic field, the wire experiences a force. If the current is reversed, so is the force. The force of the arm is directly proportional to the current flowing through the coil, achieving the required precision. Of course, when the actuator fails, we already know its consequences very well.

However, the most spectacular thing is the head. Not only does it float just above the platter (which spins at 7,200 revolutions per minute on average) at a distance of ten nanometers, but at the same time, courtesy of Faraday's law, it can detect variations in the direction of magnetic poles. Each detected “peak” symbolizes a “1”, while a “0” is interpreted where no peaks are recorded.

Reducing the distance between the head and the platter (so smooth it has a roughness of one nanometer) allows increasing the density of the sectors, and therefore the capacity of the drives. The video explores even more details, but stopping for a moment to see how a hard drive works makes it partially unfair that they go unnoticed most of the time.