Inside a hard drive

Have you ever wanted to see what the inside of your hard drive looks like? From the outside it may look like a small, simple box-shaped device, but there is an immense amount of technology inside. Read through our article which details the internal components of a modern hard drive to get a better understanding of how your data is stored, as well as an insight into data recovery.

In order to make this information more relevant we will use a new 1TB Western Digital 3.5″ hard drive, a WD10EARX.

The main enclosure, usually black in colour, is what the everyday computer user will know to be the hard drive. This is called the hard disk assembly, or HDA. The top side of the HDA is covered by the top cover, usually silver in colour, with a sticker detailing information about the hard drive such as manufacturer, capacity, model, serial number etc.

Hard drive western digital

The underside of the hard drive houses the printed circuit board, or PCB. This is the green electronic board which covers about half of the underside of the drive. The components of the PCB are usually on the inner side of the PCB, protecting them from damage. Some manufacturers, such as Samsung, place the components on the outer side of the PCB where they can be easily damaged. In this Western Digital example, the components are safely on the reverse side of the PCB. The base of the motor spindle is also shown. This contains a bearing around which the platter/s can spin inside the HDA.

Hard drive bottom with PCB
SATA drives have 2 connectors. The small connector carries data to and from the drive, whilst the larger is the power connector for 5v and 12v lines.

Hard drive connections

When removed from the hard drive, the underside of the PCB reveals the working components. There are 3 main components of the PCB. They are the main controller unit, or MCU; motor controller and onboard memory (cache).  The MCU is equivalent to the processor (CPU) of your computer, essentially the brains and computing power of the drive. The motor controller performs the function of spinning up the platter/s inside the drive, and controlling the movement of the voice coil which directs head movement. The memory chip is like the RAM in your computer. It is labelled as ‘cache’ in the hard drive world and stores data being written to, or read from, the drive. The contacts for the heads and motor can also be seen. The ROM information is also found on the PCB. Sometimes this is found on an 8-pin IC (chip), other times this information is stored in the MCU itself. In this example the ROM is stored on the 8-pin IC above the onboard memory. The ROM contains a portion of the firmware required to start the drive up, whilst the rest of the firmware is read from the drive platter itself.

Hard drive PCB

The underside of the lid houses a seal which protects the drive from the outside world. The top lid is usually secured by T8 screws, or T6 in the case of 2.5″ laptop drives. Opening a hard drive in any environment other than a certified clean room will contaminate the drive’s internals and spell disaster.

Hard drive lid

Here we have the inside of the HDA. This view shows the main components of the drive. The platter, or platters, store your data. They are usually made from aluminium or glass and are covered in several layers of other materials. The platters will spin anywhere between 5,400 RPM to 7,200 RPM in average consumer hard drives. The head stack assembly, or HSA, is the assembly which holds the read and write heads. These heads fly nanometers over the surface of the drive on sliders, reading and writing your data. The magnets allow the voice coil to function, allowing the HSA to move and is complemented by a bottom magnet below it. A plastic adapter holds the heads contact in place, making contact with the PCB on the other side. An internal air filter collects any contamination that might reside in the drive such as oil or metallic fragments, all on a micro scale. A plastic ramp is included to hold the heads in place whilst the drive is not in use. Some drives park the heads in the middle of the platters on a special layer, this being seen mainly on older drives.

Hard drive internal components

The air filter purifies the air circulating in the drive whilst in operation. It will collect any microscopic fragments of metal or oil which are used in the manufacturing process, or created through wear and tear. A hard drive which has suffered a head crash will have a filter tainted with dark material and appear black or grey. This dark material being the particles of platter/s and head slider/s.

Hard drive filter

With the top magnet removed, the voicecoil and bottom magnet are visible. The concept of this is the same as the driver in a speaker system, allowing quick and precise movements in either direction. The magnets are neodymium, the strongest form of permanent magnets being able to hold 1,000 times their weight. A stopper, or limiter, is placed at each end of the voicecoil to limit its range of movement. The bearing on which the HSA moves is also shown.

Hard drive magnets

A plastic connector is used to hold the contacts for the heads in place. They make contact with the relavant pads on the PCB on the other side.

Hard drive components

The heads connector has a rubber seal to prevent any contaminants from entering the HDA.

Hard drive head connector

The HSA holds the actual read and write heads at the end of the assembly, fixed to sliders. This particular drive has 6 heads, 3 pairs of 2. For each side of the platter there is one read and write head.

Hard drive heads parked

The sliders allow the head assembly to fly over the platters, only a few nanometers above the surface. This is many times thinner than the thickness of a human hair. When the drive is spinning at full speed, the heads will be unloaded from the parking ramp and fly over the platters. The air passing under the sliders at this speed is enough to lift them above the surface of the platters.Hard drive sliders

The rectangular black objects on the end of HSA are the sliders, whilst the actual read/write head elements themselves are so small they are hard to see without a microscope.

Hard drive head actual element

The read and write heads work with very weak signals. For these signals, the “ones and zeros”, to be of use the signals need to be amplified. The preamp on the HSA performs this feature, much like the amplifier in a hifi. It amplifies the signals from the head elements to a useable level. When a hard drive suffers a head failure, it is often the case that the preamp has failed whilst the actual heads are still functional. A head failure usually causes a ‘clicking’ sound as the heads try to position them selves, but fail and hit the limiters.

Hard drive head preamp

This summarises the main components inside a current hard drive. The goal of this article is to give a basic overview of the components inside a hard drive. Information regarding the actual workings of a hard drive is outside the scope of this article and is highly complex.

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