What is inside a Hard Disk Drive?

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For over 60 years, the hard disk drive (HDD) has played an important role in the modern era of digital technology. Storage capacity of the HDD has grown from a mere 5 MB in 1956 on fifty 24-inch disks to more than 10 TB (Tera Bytes) stored on three 5 1/2-inch diameter disks. During this relatively short period, mechatronics and control played a vital role in rapid growth in the capacity of HDD and continuously decreasing cost.

The term “mechatronics” describes a branch of engineering as integration of mechanical engineering with electronics and control in the design and manufacturing of product process. Many devices and systems we find in everyday life, such as a camera with auto-focus and auto-exposure, an automatic cash machine, a scanner or printer, a robot, and an HDD are most obvious examples of mechatronic systems. In HDDs, information bits are stored in concentric data tracks on a rotating disk coated with magnetic media while the information is recorded, as well as retrieved using the read/write head(s). In this article, we will try to describe components used in any HDD which can be broadly classified into 4 categories:

  • Magnetic components
  • Electro-mechanical components
  • Mechanical components
  • Electronics

Magnetic components

The magnetic components, the media (platter) and the head(s) are the principal components that enables storage and retrieval of binary information.


Data is recorded on a continuously spinning platter, a disk made of aluminum or glass and coated on both sides with a thin layer of magnetic material. The platter is mounted through a hole at the center on the shaft (spindle) of a motor that spins the disks. In most HDDs, the disks are spun at 5,400 or 7,200 RPM, or beyond, in the case of high performance drives. Platters are coated with several layers of other materials. Two separate elements, the write and read heads, are used for writing data to or reading data from the disks. These two heads are mounted together on a larger structure, called the slider. The slider provides electrical connectivity to both heads, helps to place the read and write heads in close proximity (flying over) to the magnetized surface of the spinning disk with its aerodynamic surface to achieve the desired flying characteristic. The air moving along with the spinning disk and en-trained between the disk and the slider’s aerodynamic surface produces an air bearing that makes the slider float. The surface of the disk must be extremely smooth to produce uniform read back signal from the heads flying a few nanometers above the disk. This is why Data Analyzers utilizes a cleanroom facility, a special environment in which HDD’s components can be examined and, if necessary, serviced or replaced (transplanted).

This is likely a good point to explain a relatively common issue related to hard drives, head stiction. A sugar grain sized head can sometimes “land’ onto the disk. While stationary, the head mounted on a slider touches the disk surface and a static-friction force is produced. This force between sliders and disks opposes the applied torque during the spinning up of the spindle, and the disk is not able to operate or spin.

Some earlier drives used to park their heads onto an appropriate texture on a small annular ring, known
as landing zone, on the disk near the center hole and sliders are pushed to this landing zone before the spindle is spun down so that they rest on the textured surface when the drive is spun up next time. Nowadays, the Dynamic Load/Unload design, has been adopted to solve the problem of stiction between head and platter as this method avoids contact between sliders and stationary disks by bringing the sliders out of the disk surface prior to spinning down. A lift tab extending from the arm engages a ramp structure as the actuator moves beyond the outer radius of the disk. The ramps lift (‘unload’) the heads from the disk surfaces as the actuator moves to the parking position. During spin up, the actuator arm is pushed over the ramp after the platter attains the specified speed so that the sliders fly.

Electro-mechanical Components

Two different electromechanical components are used in a hard disk drive: a spindle motor to spin the disk or disks and an actuator to re-position the read-write heads on the desired data track and to maintain its position precisely over the track, while data is being read or written.

Spindle Motor

A brushless DC motor is used to spin the stack of disks in an HDD. High-end, high-performance
drive spindles spin at 10,000 RPM or greater; while many models of drives for the desktop and mobile computers still use spindle speeds of 5400 RPM or 7,200 RPM.

Head Stack Assembly or Actuator (HSA)

Movement of the Read/Write head between different tracks of the disk surface is performed by an actuator. While actuators used in early days were designed around cables and pulleys, such designs were replaced by hydraulic actuators introduced in the late sixties and early seventies. Some of the early generations of HDD also used stepper motors as actuators. With increasing density of data tracks, open loop control failed to perform and closed loop control using Voice Coil Motor, replacing the stepper motors. The VCM is a moving coil type actuator, in which a coil is held suspended in the magnetic field produced by pairs of permanent magnets fixed to the casing of the HDD.

The actuator arms are made of solid steel or aluminum of significant thickness, not suitable for holding the read-write heads over the disk surface. An extended arm, known as the suspension, carries the head slider. The suspension is made of thin sheet of stainless steel and is attached to the actuator arm.
In modern drives, the HSA making the movement of the slider into an arc and not a straight line. Each head slider is attached to the tip of a suspension. Usually, there are as many sliders in a HDD as the number of platter surfaces. All suspension arms are attached to a single piece of actuator. The slider floats over the spinning disk which has a certain degree of roughness relative to the fly height. The suspension provides a force on the slider in the direction into the disk to counteract the upward aerodynamic forces of the air bearing surface. HSA usually houses a so-called commutator chip and is mounted on the flex-cable that carries a signal between heads and drive electronics.

Heads are mounted on each tip of the actuator arm/slider and are the size of a sugar grain. While technology behind this component is fairly complex and improved constantly, their internal structure and design may be explained in detail in some other article. It crucial to understand there are two major components responsible for internal drive operations platters and heads.

Mechanical Components

The drive enclosure is the external casing of the hard disk drive. It provides features for mounting the drive in the drive bay of the host system, and supports other components of the HDD, such as spindle motor, actuator, filters, electronics, etc. There are two basic parts of the enclosure – top cover and base plate. All components are assembled on the base plate. Enclosure is then covered or sealed using gaskets mounted on the top cover. The environment inside the enclosure must be maintained and clean. Any particle at the head-disk interface can cause abrasion of the disk, resulting in loss of data and increase in number of residue particles. As described above, this is why we work in a cleanroom – to ensure particle-free operations. Particles created during the operation of drive by sudden contact between disk and slider are thrown out of the spinning disk by the centrifugal force and eventually trapped in the filter, placed in the empty space inside the enclosure.


Electronic components of an HDD can be categorized according to the following functions:

  1. Electronics for reading/writing, also known as the channel electronics
  2. Electronics for spinning the disks and positioning of the read/write head, also known as the servo channel
  3. Electronics for controlling various operations (such as read data, write data, transfer data between HDD and host etc.) of the disk or the disk controller
  4. Electronics for interface with the host system, and
  5. RAM, ROM etc.

Several of these functional components are often combined in a single chip (ASIC). As a result, we do not see many ICs on the PCB (printed circuit board) of an HDD. As stated above, one IC that is not placed on the PCB, but is kept inside the drive enclosure is the pre-amplifier (or commutator).

While we tried to cover different situations in which clients lost their data on a hard drive and successfully recovered using Data Analyzers, this article is here to help you distinguish each component in a hard drive as it may be mentioned in your analysis report made by our engineers upon receiving your drive.

Have fun computing and don’t forget to back up!

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