How a hard drive works

A hard drive is an amazing piece of mechanical engineering. Every hard drive contains one or more spinning metal platters, which hold the magnetically stored data. Data is read and written by an electromagnetic head, which moves across the platter on an arm.

On an interesting side note, the term “crash” harks back to the days when hard drives were as big as a fridge-freezer, and the head would often plough into the spinning disk, causing the drive to fail. The head runs so close to the platter that a greasy fingerprint on it is enough to make the head crash. Even a single particle of cigarette smoke is large enough to cause problems. This is why hard drives are completely sealed to keep dust out. Opening up a hard drive or taking it to pieces will almost certainly destroy the drive no matter how careful you are.

That said, lets break out the screwdrivers and take one to bits. Our victim is a 30GB Fujitsu ATA100 drive, which is a relatively modern 2 platter design.

You can see here the spinning platters on the right, with the arm on a fixed pivot.

As the photos show, both sides of each platter are used, but all the heads are mounted on the same arm and have to move together on the fixed pivot. This means that no matter how many heads a drive has, only one of them can be working at once.

Having more platters will tend to slow the drive down, since the arm is heavier with the increased number of heads. It now takes longer to move from one side of the disk to the other, and the heads have to move further and more often to find scattered data. A larger hard drive can be made by adding another platter to an existing design, or by increasing the density of data on the platters. Increasing data density makes the drive faster, because it increases the amount of data you can get per revolution of the platter.

However, having more platters isn't always slower. As all the heads move together, they write data in columns. With 4 double sided platters, a whole byte (8 bits) can be written at the same time, making the data less scattered.

S.M.A.R.T.

S.M.A.R.T is a little known feature which has been included as standard on almost all hard drives for more than 5 years now. It is a system which is intended to predict hard drive failure by monitoring the drives vital signs. This includes recording error rates, drive response times, and most importantly, drive temperature. You may not have realised it, but it is likely your hard drive has its own inbuilt temperature sensor for this very purpose.

Hard drives run a much higher risk of failure once they get hotter than 60 degrees, and drives stacked up in a poorly ventilated case, or enclosed in sound damping material can easily reach these temperatures. To access the sensor, you will need a utility such as HD Tune 2.1.

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