Anatomy of a hard drive

Anatomy of a hard drive

Hard drive Data Recovery techniques are improving all the time, yet the fundamental components of a hard drive have remained the same since their commercial introduction in the mid to late 80s.

This article will explore the key components and functionality of a hard drive, explaining the terminology and highlighting the technology with additional data recovery context. It’s a brief insight into how your data is stored and the types of faults we see every day as a data recovery company.

Hard drive components

Hard drive diagram

Hard drive casing

The main casing of a hard drive is typically cast aluminium with a stainless steel lid. This allows it to be lightweight yet as rigid as possible to protect the internal moving parts. Even a small flexing of this case during operation could cause serious problems.

Disk platters

The disks in a hard drive are called platters and this is where your data is stored. There are typically between 1 and 5 platters stacked on a central spindle, with data being stored on both sides of each disk.

Each platter is made of either aluminium, glass or a ceramic material. The platters are coated in a number of layers of different materials, the most important of these being a magnetic layer. This layer is a covering of tiny magnetic grains, these grains form microscopic areas that can be magnetized to represent a 1 or demagnetized to represent a 0. These 1s and 0s are then translated into the information that you interact with when you use your computer. This information includes the computers operating system, software and all the other data that we are so familiar with.


The platters are mounted on a central spindle that is rotated by a spindle motor. This is a small high precision electric motor designed to be incredibly reliable and fast over long periods of time. The disk must rotate without any vibration or wobble, any kind or imbalance can easily cause read/write errors on the disk.

Hard drive spin speeds vary from 5,400rpm to 15,000rpm, however most consumer hard drives spin at 7,200rpm.

Actuator Arm

The actuator arm moves the read/write heads back and forth across the surface of the platter. It pivots on a spindle at one end and houses the head assembly at the other end. If there are multiple platters in the hard drive assembly then there will be multiple arms that work in unison to read and write data on both sides of each disk platter surface.


The actuator unit is responsible for moving the read/write arm. This is achieved by an electromagnet called a voice coil (or actuator coil) which is on the pivoting arm on the opposite side of the actuator spindle to the read/write heads. This works together with a fixed magnet assembly, using electromagnetic attraction and repulsion to move the read/write arm in a rapid, smooth and precise movement.

Read/Write heads

As the read/write heads move across the surface of the platter they read the magnetic surface layer of the disk and convert this into an electrical current which can then be interpreted by software in the computer. Writing data is the opposite where an electric current is used to change the magnetic field on the required areas of the disk.

The heads never touch the surface of the disk as this would cause physical damage and make the data unreadable. The heads have a ‘flying height’ of around 3-6nm (Nanometres) above the disk surface and glide over a cushion of air. To give you an idea how close this is, there are 1,000,000 nanometres in a millimetre.

When the device is switched off the read/write arm moves the heads to a ‘parked’ area away from the disk surface, this prevents the heads from contacting the disk surface in the event of an impact.

Many hard drives have a mechanism in place to detect if the device is falling and quickly moves the heads away from the disk surface. Quite often a fall from a greater height is less likely to cause damage than impact from a relatively short height.


A printed circuit board known as a Controller Board allows power to the relevant components, it controls the functionality of the drive and allows communication between the drive and the device it is connected to.

It has a connector that allows the hard drive to be attached to a controlling device (e.g. PC, Laptop), this connector is either IDE, SCSI, USB or SATA. The majority of modern hard drives use SATA connectors.

Air filter

In order for the heads to float correctly over the surface of the platter, the air pressure inside the casing needs to be equal to the air pressure outside of the casing. To achieve this there is a small air inlet vent. This vent is covered by a filter to prevent tiny particles entering the casing and potentially getting onto the disk surface. Even a small particle on the surface could cause damage if hit by the read/write heads.


As you can see, hard drives are incredibly complex pieces of technology. They are designed to very high specifications and thoroughly tested throughout the manufacturing process.

The technology has improved exponentially over the years, however with an ever increasing requirement for larger capacities whilst maintaining fast data access, there is also a higher potential for failure.

If you experience any problems accessing your data, switch the device off and to do not attempt to use it until you have spoken to a data recovery specialist