What are the functions and parts of the hard disk drive?

A hard disk drive (HDD) is a data storage device used in computers that contains one or more spinning platters coated with a magnetic material for recording data (TechTarget, 2022). HDDs were first introduced by IBM in 1956 and have been the dominant form of non-volatile data storage in computers for decades.

HDDs store and retrieve digital data using one or more rigid metal platters coated with magnetic material. The platters are mounted inside a metal casing and spun at high speeds. Moveable read/write heads float just above the platter surface detecting and modifying the magnetization of the material to record data (Crucial, 2022).

Data is written to and read from the platters using the read/write heads, which are mounted on an actuator arm. The arm moves the heads radially across the platters as they spin, allowing data to be accessed on the entire surface area of each platter. The data is organized in concentric tracks divided into sectors. HDDs use magnetic induction to read/write data, unlike solid state drives (SSDs) that use electron storage (Wikipedia, 2022).

Main Components

A hard disk drive consists of several key components that work together to store and retrieve data. According to the Chron, the four major internal components are:

  • Platters – The disks inside the drive that actually hold the data. Hard drives typically have multiple platters stacked on top of each other. The data is written magnetically to both sides of the platters.
  • Read/Write Heads – The heads float just above the drive’s platters on an air cushion, reading and writing data. There is one head for each platter surface.
  • Actuator Arm – The arm that holds all the read/write heads and moves them across the platters as needed to access data.
  • Spindle – The spindle spins the platters at very high speeds, typically 5400 to 15000 rpm.

According to HDDscan, other key internal components include:

  • Casing – The metal or plastic housing that protects and holds all the parts together.
  • Controller board – The circuit board that manages the hard drive’s operations.
  • Firmware – The hard drive’s operating system, controlling the reading/writing of data.

Together, these components work to reliably and quickly store, locate, and retrieve digital data magnetically encoded on the drive’s spinning platters (HDDscan).


Platters are the disks inside the hard drive that store the data. They are made of non-magnetic material like aluminum, glass, or ceramic and are coated with a thin layer of magnetic material like cobalt-based alloy (Wikipedia, SuperUser). The coating allows data to be stored on the platters magnetically. Typically there are several platters stacked on top of each other inside the hard drive. The magnetic coating on each platter is organized into tiny magnetic regions that can be magnetized in different directions to represent 0s and 1s to store data. The platters rotate at high speeds while the read/write heads float nanometers above them, accessing data as needed.

Read/Write Heads

The read/write heads in a hard disk drive are responsible for writing data to the platters and reading that data back. They hover above the platters and transform each platter’s magnetic field into electric signals to read the data. To write data, they magnetize tiny spots on the platters by generating a strong local magnetic field. The read/write heads are affixed at the end of an arm, allowing them to move across the surface of the platters while maintaining a consistent hovering height of just nanometers above the platter surface. This prevents the heads from making contact with the platter surface and damaging the data (Wikipedia, Techopedia).

Actuator Arm

The actuator arm, also called the access arm, carries the read-write heads and moves them across the platters as they spin. This allows the heads to access almost the entire surface of each platter. The arm is made of lightweight but rigid material to provide fast and precise positioning of the heads

A voice coil motor drives the movement of the actuator arm. It uses a coil of wire and a magnet to convert electric signals into the back-and-forth motion to position the heads. The voice coil design allows very fast starts, stops and transitions, enabling quick access to data anywhere on the drive.

The arm swings the heads in an arc across the platters as they rotate. The precision of the voice coil and rigidity of the arm keep the heads at just micrometers above the surface to read and write data. This close positioning is critical to allowing today’s high areal density drives.


The spindle is the mechanism that rotates the hard disk platters inside the drive enclosure. It consists of a spindle motor and a spindle shaft (Hard disk drive performance characteristics). The spindle motor spins the platters at high speeds, typically between 5,400 RPM to 15,000 RPM in modern hard drives. The spindle shaft is attached to the spindle motor on one end and fits through the center of each platter on the other end. This allows all the platters to spin in unison at the same speed.

The rotational speed of the spindle is one of the main factors that determines the performance of a hard drive. Faster spin speeds allow the read/write heads to access data on the platters more quickly. However, higher speeds also generate more heat and vibration, consume more power, and reduce the drive’s lifespan. Most consumer hard drives today use 7,200 RPM spindle speeds, while high-performance models may use 10,000 RPM or even 15,000 RPM.


The casing of a hard disk drive serves to protect and seal the delicate internal components from outside elements like dust and moisture. It is usually made of aluminum or a sturdy plastic and consists of a top and bottom half that are screwed together during assembly. The casing has mounting holes on the bottom and side to allow the drive to be secured into a computer case or external enclosure.

The casing maintains normal air pressure and humidity inside the hard drive to prevent issues like condensation and head crashes. It also helps dampen vibration from the spinning platters and actuator arm movement that could otherwise disturb the precise operation of the read/write heads. Some high performance drives even fill the casing with helium due to its lower density compared to air which reduces drag on the spinning platters. While hard drives are better equipped to withstand shock when powered off, the casing still protects the platters and arm from damage if the drive receives an impact while running.

Most external hard disk drive enclosures also have a casing to enclose the internal drive, protecting it from the elements and adding structural rigidity. The external enclosure casing will have USB, Thunderbolt, or other data/power ports to connect the drive to a computer instead of SATA ports. Some larger multi-bay enclosures are even designed to be rack mounted for mass storage applications.[1]

Data Encoding

Data is stored on a hard disk drive using magnetic encoding. The platters are coated with a magnetic material and the read/write heads change the magnetization of a small area on the platter to record data (Stanford).

The platters are organized into concentric tracks which are further divided into sectors and clusters. Tracks located closer to the edge of the platter can store more information than tracks closer to the center. To optimize storage capacity, the tracks are organized into zones with each zone storing a set number of sectors (Engineers Garage).

A sector is the smallest unit of storage and typically stores 512 bytes of user data. Multiple sectors are grouped together into clusters which is the smallest amount of disk space that can be allocated to store a file. The number of sectors per cluster varies based on the operating system but is commonly 8 sectors per cluster (Stanford).

Performance Factors

Three key factors determine the performance of a hard disk drive: spindle speed, seek time, and data transfer rate. Spindle speed refers to how fast the platters rotate inside the hard drive, measured in revolutions per minute (RPM). Common spindle speeds are 5,400 RPM for lower-performance drives and 7,200 RPM for mainstream desktop drives, though 10,000-15,000 RPM drives are available for high-performance needs. A faster spindle reduces the time needed for the read/write heads to access data on the platters (Wikipedia [1]).

Seek time refers to how long it takes the actuator arm to move the read/write heads to a specific location. It is measured in milliseconds (ms) and includes the time to move the arm and settle on the track. Seek time tends to range from 2-10 ms for modern hard drives. Drives with lower seek times can access data faster (Build Computers [2]).

Finally, data transfer rate measures how fast data can be read from or written to the drive, and is affected by factors like the disk’s rotation speed and recording density. Data transfer rate is measured in megabytes per second (MB/s) or gigabits per second (Gb/s). Faster hard drives achieve rates of 150-200+ MB/s for sequential reads and writes.

[1] https://en.wikipedia.org/wiki/Hard_disk_drive_performance_characteristics
[2] https://www.buildcomputers.net/hard-disk-speed.html

Failure and Lifespan

Mechanical hard drives have several failure modes that can cause data loss or drive failure over time. The most common failure modes include:

– Head crashes – where the read/write head makes contact with the platter surface, damaging the thin magnetic coating.

– Motor failure – where the spindle motor stops working, preventing the disk from spinning up.

– Contamination – things like dust or smoke particles inside the drivecase that interfere with the heads reading/writing data.

– Wear and tear – things like bearings wearing out over time as platters spin.

According to Backblaze’s statistics on over 100,000 hard drives, the annual failure rate for consumer hard drives is around 1.5% per year[1]. This means about 98.5% of drives survive each year. Hard drive lifespan is estimated between 3-5 years on average before mechanical failure occurs.

To monitor hard drive health, the S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) system checks and reports on various drive attributes and alerts users to impending failures. However, not all failures are predicted by S.M.A.R.T.

Regular backups are still essential to protect against sudden drive failures causing data loss.

[1] https://www.backblaze.com/blog/backblaze-drive-stats-for-q1-2023/