What is the hard drive explained?

A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage and one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces.

What is the purpose of a hard drive?

The primary purpose of a hard drive is to store data. It provides long-term storage capacity so you can save files, install software, and more. The hard drive is a permanent storage device located inside your computer.

Some key uses and purposes of a hard drive include:

  • Storing your operating system – The hard drive stores the operating system files like Windows, macOS, etc. It needs this to boot up and load the OS.
  • Installing software and apps – Programs and software get installed to the hard drive so they are permanently accessible.
  • Saving files and documents – All your personal files like photos, videos, documents, etc. get stored on the hard drive.
  • Caching data from slower storage – It can act as a cache storing frequently accessed data from optical drives and networks.

In summary, the hard drive provides high-capacity permanent storage for the OS, apps, files, and data on your computer or laptop.

How does a hard drive work?

A hard drive works by using magnetism to store and access data on a series of spinning disks called platters. Here is a high-level overview of how a hard drive functions:

  1. The platters consist of an aluminum or glass substrate coated with a thin magnetic layer.
  2. A read/write head floats just above each platter, attached to an actuator arm mechanism.
  3. As the platters spin very quickly, the heads can magnetically record data onto the platters or read data from them.
  4. The actuator arm moves the heads across the platters as needed to access different data tracks.
  5. A bit is written or read when the head aligns with a track and detects or changes the magnetic orientation.
  6. The data is processed through the drive’s controller and interface to the motherboard.

The hard drive relies on the coordinated motion of the platters, actuator arm, and heads to locate, access, and store data using magnetic polarities. The fast rotation allows data to be read or written quickly anywhere on the drive’s platters.

Key Components

Here are some of the key components that make up a hard drive and their functions:

  • Platters – The circular disks that data is stored on. Made of non-magnetic material like aluminum or glass and coated with a thin magnetic layer.
  • Read/Write Heads – Devices on the end of the actuator arm that can magnetize or detect polarization to read/write data.
  • Actuator Arm – Moves the heads across the platters to access different data tracks.
  • Spindle – The motor that spins the platters at very high speeds.
  • Controller – The circuit board with the electronics that handles drive operations and communication.
  • Firmware – The drive’s built-in operating system managing low-level operations.
  • Interface – Allows communication from the drive to the computer, usually SATA or SAS.

Basic anatomy of a hard disk

Here is an overview of the basic parts and anatomy that make up a typical hard disk drive:

  • Platters – The stacked disks that data is written to. Made of non-magnetic material and coated in magnetic layer.
  • Spindle – The center rod that platters spin around. Connected to motor below.
  • Actuator arm – Holds the read/write heads and moves them across platters.
  • Actuator axis – Pivot point that actuator arm rotates around to access platters.
  • Head assembly – Read/write heads attached on sliders at end of actuator arm. Suspended just above platter surface.
  • Motor – Provides power to spin the platters at high speed.
  • Circuit board – Contains controller and interface electronics and connectors.
  • Casing – Metal housing that protects and seals internal components.

These are the core components that enable the drive to read and write data and communicate with the computer. Different form factors change the shape and size of the housing but the general internal anatomy remains similar.

How data is stored on a hard disk

Data is stored on a hard disk drive magnetically by polarizing small regions of the disk surface. Here is a closer look at how data storage works on a hard drive:

  • Each platter is formatted into concentric tracks like rings on a tree. Tracks are further divided radially into sectors.
  • As platters spin, the head floats on a cushion of air over each track to access a specific sector.
  • An electromagnetic coil in the head charges to polarize a tiny region of the disk surface under it with a magnetic field.
  • This polarization creates a binary 0 or 1, storing a bit of data. Magnetic alignment in one direction is 0, the other is 1.
  • Millions of polarizations in patterns represent all the drive’s stored data bits in binary code.
  • To read data, heads detect the magnetic polarizations as platters spin by to recreate the binary data.

The drive controls the precise movements of heads and platters to access data from tracks and sectors across the disks. Data is distributed across the platters to optimize storage capacity and performance.

Tracks and Sectors

Tracks and sectors provide a logical data organization scheme on the platters:

  • Tracks – Concentric rings across each platter surface. Thinner tracks towards center allow more tracks per platter.
  • Sectors – Angular subdivisions of each track. Typically 512 byte blocks. First sector used for track/sector address identification.

Hundreds to thousands of tracks combined with tens of sectors per track allot billions of logical block addresses to locate data on the drive.

Hard disk form factors

Hard drives come in different standardized sizes known as form factors. The common hard drive form factors are:

3.5 inch

  • Desktop computer standard size
  • Metal case measures 4″ x 5.75″ x 1″
  • Requires separate power supply
  • Higher capacity drives, up to 10TB+

2.5 inch

  • Laptop standard size
  • Thinner aluminum case about 2.75″ x 3.96″ x 0.28″
  • Typically max 4TB capacity
  • Lower power needs

1.8 inch

  • Small lightweight drives
  • Case measures 2.1″ x 0.7″ x 0.18″
  • Lower capacities around 500GB max
  • Mainly used in portable external HDDs


  • Compact size for space-constrained devices like laptops
  • Various lengths up to 2.5″
  • Connects directly to motherboard through PCIe interface
  • No case or cabling needed
  • Capacities similar to 2.5″ drives

Larger form factors offer greater capacities while smaller ones trade capacity for smaller size and power needs.

Hard disk drive performance factors

Several key factors determine the performance of a hard disk drive:

Rotational Speed

  • Speed platters spin measured in revolutions per minute (RPM)
  • Faster RPM reduces seek times to access data
  • Common speeds: 5400 RPM (laptop), 7200 RPM (desktop), 10,000-15,000 RPM (high performance)

Cache Memory

  • Faster memory buffering data between drive and computer
  • Larger cache size improves transfer speeds
  • Typically ranges from 16-256 MB

Interface Transfer Rate

  • Connection interface affects maximum data transfer speed
  • SATA interfaces up to 6Gbps, SAS up to 12Gbps speeds

Average Seek Time

  • Time for head to move to track containing data
  • Lower time allows faster data access
  • Around 10 ms for 7200 RPM drives

Data Transfer Rate

  • Sustained rate drive can read/write data to platters
  • 100-200 MB/s typical for modern HDDs

Combination of these factors determine overall speed and responsiveness when loading programs or accessing files.

Hard disk drive interfaces

Hard drives use various standardized interfaces to connect to a computer and transfer data. Common hard drive interface types include:


  • Serial ATA: Common HDD interface
  • SATA revisions up to SATA III with 6Gbps transfer speeds
  • Uses narrow 7-pin data cable for point-to-point connection


  • Serial Attached SCSI: Used in servers and high-end workstations
  • SAS-3 runs up to 12Gbps dual-port connection
  • Supports multiple drives on a shared bus


  • Small Computer System Interface: Legacy parallel interface
  • SCSI and Fast/Wide SCSI standards up to 80MB/s transfer
  • Required bulky cables and terminators


  • Older parallel ATA interface
  • 133MB/s max transfer rate
  • Two drives daisy-chained per channel

Fibre Channel

  • Used in Storage Area Networks (SANs)
  • Fibre optical cabling for networks and RAID arrays
  • Very high performance with multi-gigabit speeds

The SATA interface is the de facto standard for most modern hard drives due to good speeds and simple cabling.

Internal hard drive vs. external hard drive

The main differences between internal hard drives and external hard drives are:


  • Mounted inside computer case
  • Connected directly to motherboard
  • Powered from computer’s PSU
  • Typically higher performance
  • Not easily portable or removable


  • Housed in own enclosure outside of case
  • Connected via USB, eSATA, or FireWire
  • Has own power supply
  • More portable and easily removed
  • Typically lower performance

Internal hard drives offer higher performance as they interface directly with the computer but external drives allow for easier portability and sharing between devices.

Hard disk drive vs. solid state drive

The main differences between mechanical hard disk drives (HDD) and solid state drives (SSD) are:

Hard Disk Drive (HDD) Solid State Drive (SSD)
Magnetic disk media Flash memory chips (NAND)
Platters, read/write heads, moving parts No moving parts
Slower access times due to physical movement Faster access, reads and writes
Lower cost per gigabyte currently Higher cost per gigabyte
Slower read/write speeds Much higher data transfer speeds
More susceptible to physical damage More shock and vibration resistant

In general SSDs provide much better performance while HDDs allow for lower cost mass storage. HDDs involve more delicate moving parts while SSDs are more durable.

Hard drive manufacturers

Some of the major brands manufacturing hard disk drives include:

  • Seagate – One of the largest HDD makers, produces a wide range of consumer and enterprise drives
  • Western Digital – Top hard drive brand specializing in desktop and portable storage solutions
  • Toshiba – Makes HDDs for various markets including consumer PCs, data centers, surveillance
  • Samsung – Major electronics firm that produces HDDs and flash memory
  • Hitachi – Owned by Western Digital but continues manufacturing enterprise-class drives

Some other notable hard drive makers include HGST, Micron, Intel, and various smaller brands catering to niche markets.

The future of hard disk drives

While solid state drives are taking over some storage applications, hard disk drives will continue playing a major role in the future due to advantages in cost and capacity. Some trends for the future of HDD technology include:

  • Increasing areal densities for higher capacity drives beyond 10TB+
  • Use of shingled magnetic recording (SMR) to squeeze more data on platters
  • Multi-actuator arms to access platters concurrently rather than sequentially
  • Helium filled drives to reduce friction and power use
  • Heat-assisted magnetic recording allowing further miniaturization
  • Hybrid drives fusing HDD capacity with SSD speeds

Hard drives will co-exist with SSDs for mass storage needs. Enterprise and data center usage will drive innovation in HDD technology and capacities for years.


In summary, the hard disk drive is permanent magnetic storage that provides long-term data capacity for computers. It works by using platters, actuator arms, and heads to read and write data magnetically at high speeds. Key factors like RPM, cache size, and interface speed determine drive performance. While SSDs are faster, HDDs continue evolving to offer greater capacities for mass data storage needs.