How Does A Hard Disk Work ?

Seeing as this whole site is about hard drives and what to do with them when they go wrong, I thought it would be a good idea to describe what’s inside them and how they work. Enjoy !

Inside a hard disk driveDid you know that your hard drive is the slowest part of your computer? It’s true – ironically, the computer’s most important component – the part that stores all your valuable data is also the part that holds up the entire computer from operating as fast as it could. The reason it’s so slow is because it’s a mechanical component with moving parts, and this movement takes time.
Thanks to technological innovation, hard drives have got faster, quieter and consume less power than they used to. Also, the quantity of data they can hold has increased exponentially – this is referred to as areal density.

Explaining more about Areal Density…
This is the amount of data that can be stored to a square-inch of platter space. Each platter stores information in tracks. You can think of the tracks as circular rings that are etched on the platter. And onto these rings, the data bits are recording linearly. The more bits that are packed along the tracks, the more data can be stored. So why can’t hard disk manufacturers keep packing more and more onto each square inch of hard drive? The answer is because of both physical and electromagnetic limitations. To make a disk platter, you have to take a virtually featureless disk (meaning that it has no markings, particles or dents on it – perfectly smooth) and embed microscopic magnetic marks on it. To increase areal density, you have to make those marks smaller in size and decrease the size of the read/write device. But the smaller the bits, the harder they are to be read because of signal intensity.

What’s inside a hard disk drive?
A hard disk drive comprises of many parts, all of which are sealed in a dust-free chamber to ensure correct drive operation. The basic components of a hard drive are:

The Platter(s) – These are metal alloy disks that are coated with either an aluminium alloy or glass substrate. This is where the hard drive stores information. All your data is stored using magnetic polarity differences. For example, a northern charge would denote a 1 and a southern charge would denote a 0. So the more platters a hard drive has the more data it can hold? Not quite…

The Spindle and Drive Motor – Each platter rotated by the hard drive’s spindle motor which spins the platters at a specific speed. Common spin speeds are 5,400RPM, 7,200RPM, 10,000RPM and 15,000RPM. The faster the platters rotate, the faster the read/write head can obtain data, and the more expensive the drive. The spindle motor uses ball-bearings to rotate around as they are faster and smoother than brush-less motors. The spindle is one of the most sensitive parts of the hard drive. If it receives a significant shock, a bearing may work from it’s housing causing the hard drive to fail.

The Actuator – The actuator is responsible for moving mechanical arms across the platter surface as the disk rotates and is capable of minute movements which position the read/write heads precisely to locate a specific track and sector. The actuator is one of the most important parts of the hard drive that determine the speed of the drive and how many bits it can pack on its platters. The actuator must align the read/write head precisely, if not, the data will not be read or written correctly. The read/write head is attached to the ends of the actuator arms. It looks like the heads are touching the platters. But in actuality, they are “flying” above the platters using a cushion of air created by the spinning platters. This thin layer of air is what prevents the heads from coming in contact with the platters. An excessive shock to the drive during operation can the heads to ‘crash’ into the platter surface – often resulting in a head crash.

The Read-Write Heads – The heads are responsible for reading and writing data to the drive. They write data by arranging the magnetic particles on the surface of the platters. When arranged in one direction, the particles will represent a 0 and when arranged in the other direction, they will represent a 1. When reading from the platters, the head will detect the polarity of the particles and translate that into electrical signals and send the signals back to the on-board hard drive controller.

The Controller Board – The hard drive controller is the printed circuit board that is attached to the hard drive chassis. The controller contains several chips that regulate the power and control timing and dataflow across the drive. It also contains chips which cache data from the drive that is used often. Inside the hard drive is an additional printed circuit board that controls the actuator.

Magnets – A hard drive contains two very strong magnets located inside the actuator. One is on top of the actuator arms and the other one is underneath. The reason why these magnets do not affect the platters is that the magnetic field on the magnets lie on a vertical axis. If they were on a horizontal axis, the field would stretch onto the platters, destroying the data. Another reason is that the magnetic signals recorded on the platters aren’t any regular magnetic fields but they are electro-magnetically recorded. Electro-magnets are many times more stronger than natural magnets.