How a Hard Drive Works

Even if you’re a self-proclaimed computer geek who can build a PC from components, I bet there are parts that you don’t fully understand. You might know, in abstract terms,how a computer restarts — but do you know the actual process? Likewise, there are few people — geek or otherwise — who don’t know the basis of how a hard drive operates; disks spin and heads read and write data.. but beyond that, how much do you know about the actual operation of a hard drive?

How do the heads move, for example? The head must be able to accurately seek a magnetic region that is just a few nanometers wide while the disk spins at thousands of revolutions per minute. That certainly couldn’t be done with a normal motor — and in fact, hard drive heads are moved with a voice coil actuator, much in the same way that a cone in a speaker is moved to make sound. By applying tiny amounts of electricity to a wire, a Lorentz force is used to move the hard drive head very accurately. There are no cogs, and minimal wear and tear — which is one of the reasons why hard drives last for such a long time (compared to other machines that operate under such conditions, anyway).

How does a head read data? At its most basic, the head is a piece of metal that’s wrapped in wire. As the head moves over the magnetic fields on the platter, changes in magnetism induce a current that is measured and converted into a binary value. It’s not quite that simple — there are different ways of making hard drive heads and encoding data on the magnetic surface — but Faraday’s law of induction is always used.

Finally, because the density of data is so high on modern drives — up to 625 billion bits (78GB) per square inch on a 1TB platter — the head must float just 5 or 10 nanometers above the magnetic regions. Instead of trying to machine a fixed head that hangs 10nm above the platters, modern hard drive heads float on a layer of air that’s created by the rotation of the drive. This technique is self-correcting: if the head rises too much, it loses buoyancy and falls back down to its “floating height.” Just so you have some idea of how close a head flies over a hard drive platter: 10nm is three times smaller than the transistors used in the latest computer processors — and as hard drive density increases, the floating height will only get lower.

For more information, watch Bill the Engineer Guy in the video below. Everything we’ve covered here is in the video, plus a few more cool factoids.