What is a hard drive and what is it used for?

A hard drive is a data storage device used in computers and other electronic devices to store digital information. It is a non-volatile storage device, meaning it retains data when power is turned off. Hard drives come in different storage capacities and sizes and are one of the most common types of secondary storage in modern computers.

What does a hard drive do?

The primary function of a hard drive is to store data that can be quickly accessed by the computer’s operating system or other programs. This includes the operating system itself, software applications, user documents, games, media files like photos, videos and music, and any other files that need to be saved.

Hard drives allow computers to have fast access to large amounts of data compared to other forms of storage like optical discs or tape storage. The operating system can read and write data to and from the hard drive much faster than retrieving data sequentially from a disc or tape.

For most desktop and laptop PCs, the hard drive is the main storage location for almost all data. It provides consistent performance for random access to data, unlike sequential access from optical media like CDs or DVDs. This makes it ideal for storing the programs and files that the computer uses actively.

What are the components of a hard drive?

A hard drive consists of several key components enclosed in a metal case:

  • Disk platters – Usually made of aluminum or glass, these spin at high speeds of 5,400 to 15,000 rpm in the drive. The surfaces of each platter are coated with a magnetic material that stores the data.
  • Spindle – The rod that rotates the platters at high speeds.
  • Read/write head – The head floats just above the disk on an air cushion, reading and writing data. One head is used for each disk surface.
  • Actuator – The mechanism that moves the heads to locate data on the disk platters.
  • Controller – The circuit board with the electronics that control the drive operations and transfer data to and from the computer.
  • Firmware – The logic programmed onto the controller to manage the operation of the drive.
  • Casing – The metal casing that encloses and protects the internal components.

Hard drives have a stack of disk platters that rotate at high speeds. The read/write heads access data on the platters while they spin. The platters, read/write heads and actuator are collectively called the head disk assembly. The electronics board with the controller and firmware provides the interface between the drive and computer.

How does a hard drive store data?

Hard drives store data using magnetic recording techniques. Each disk platter has a magnetic coating. The read/write head changes the magnetic orientation of tiny areas on the disk surface to represent binary 1s and 0s just like any magnetic storage medium.

The surface of each platter is divided into billions of tiny regions called bits. Each bit can be magnetized in one of two directions to store a 1 or 0. All the bits together make up the sectors and tracks where data is written.

The drive writes data by positioning the head over the correct track and magnetizing bits in a pattern that encodes the data. When reading, the head detects the magnetic orientations to read the original pattern. The controller translates the bit patterns back into binary data.

The concentric tracks are divided into logical block addresses or sectors that store a fixed amount of data, commonly 512 bytes in modern drives. This organization allows the drive to efficiently access any part of the disk by moving the head to the correct track and then waiting for the right sector to reach it.

Hard Drive Capacities

Hard drive capacities have grown enormously over the years as drive technology continues improving. Some key advances enabling larger capacities include:

  • Increasing areal density – More bits per square inch on the platter surface, achieved through lower bit sizes.
  • More platters – Increasing the number of disk platters to add more recording surfaces.
  • Higher efficiency encoding schemes – Cramming more bits in the same space.

Today’s consumer hard drives typically have capacities up to 10TB for desktop models and 2TB for laptop models. Enterprise and specialty drives can go much higher – 20TB or more. drive size & technology factors.

Year Drive Capacity
1956 3.75MB
1980 10MB
1990 1GB
2000 20GB
2010 2TB
2020 16TB

This table shows the rapid growth in hard drive capacities over the decades, from just a few megabytes initially to multiple terabytes today.

Hard Disk Interfaces

Hard drives use different interfaces to connect to the computer and transfer data. Common hard drive interfaces include:

  • PATA – Parallel ATA, the original hard drive interface, used a parallel cable with many wires. It has mostly been replaced by SATA.
  • SATA – Serial ATA uses a thinner serial cable and is the most common hard drive interface today.
  • SAS – Serial Attached SCSI is used in enterprise hard drives for its higher performance and reliability.
  • FC – Fibre Channel is another enterprise drive interface known for fast speeds over long distances.
  • USB – External portable hard drives use USB, like flash drives. USB is slower than internal SATA but more convenient to connect.

Internal hard drives for desktop PCs and laptops overwhelmingly use the SATA interface today. Enterprise servers may use SAS or FC interfaces for better performance under heavy workloads. External portable drives usually connect via USB.

Form Factors

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

  • 3.5 inch – The classic hard drive size, used in most desktops.
  • 2.5 inch – Smaller drives used in laptops and some desktops. Also in external enclosures.
  • 1.8 inch – Very small hard drives for portable devices. Less common today.
  • M.2 – A compact slot-based form factor commonly used in laptops and small devices.

3.5 inch drives maximize capacity while 2.5 inch drives are smaller and more portable. Enterprise data centers may use larger special form factors.

Internal vs External Hard Drives

Internal hard drives are installed inside the computer case and directly connected to the motherboard. This includes 3.5 inch and 2.5 inch form factor drives in desktops and laptops. Internal drives offer the best performance and highest capacities.

External hard drives enclose the drive in a portable case with standard interfaces like USB. The small, portable design makes external drives easy to disconnect and move between computers. However they typically have lower performance than internal drives.

Many users add external USB hard drives for extra storage or backups, while keeping the operating system and main programs on the faster internal drive. Both internal and external drives provide useful storage options.

Hard Disk VS SSD

SSDs or Solid State Drives are the main competitor to traditional hard disk drives. SSDs have no moving parts and use non-volatile NAND flash memory to store data. This allows SSDs to be faster, quieter and less prone to damage from physical shocks. However SSDs are also more expensive and have lower capacities than hard drives.

Typical advantages of hard disk drives compared to SSDs include:

  • Lower cost per gigabyte – HDDs are cheaper for higher storage capacities.
  • Higher maximum capacities – HDDs go up to tens of TBs while SSD capacities top out at around 16TB currently.
  • Established technology – HDDs have decades of development while SSDs are newer.

SSDs have the edge in performance, power efficiency and reliability. But hard disk drives retain advantages in price, capacity and maturity. Many systems use a combination of both HDDs and SSDs to get the best of both technologies.

Main Uses of Hard Drives

Some of the most common applications of hard disk drives are:

  • Primary storage – Hard drives provide the bulk of storage capacity in computers and servers to store the OS, programs and data.
  • Servers – Hard drives store huge amounts of data in server farms and data centers.
  • Desktop computers – All regular desktop PCs use hard drives for storage.
  • Laptops – The main internal storage for laptop and notebook computers.
  • Game consoles – Gaming machines like the Xbox and PlayStation systems use specialized high performance hard drives.
  • External storage – External portable hard drives are commonly used for backup and secondary storage.
  • NAS devices – Network attached storage relies on banks of hard drives for home and office file storage and sharing.
  • DVR – Digital video recorders use hard drives to record video footage.

Hard drives are an indispensable storage technology for computers of all types due to their performance, capacities and low cost.


Hard disk drives have been the dominant secondary storage technology for computers for decades. They continue to offer a unique mix of performance, huge capacities, and low cost per gigabyte compared to solid state and optical storage. With consistent development and improvement over many years, hard drives are now available in multi-terabyte capacities with fast access speeds.

While SSDs are faster, hard drives still excel at providing the most storage for the money. Their mechanical operation with moving parts results in some inherent limitations in speed and reliability compared to flash drives. But ongoing advances in areas like areal density mitigates some of those weaknesses while building on their inherent strengths for capacity and affordability.

For applications where massive amounts of data need to be stored at the lowest possible cost, traditional hard disk drives are still the top choice. Their expected dominance for bulk storage is likely to continue into the foreseeable future despite challenges from alternatives like SSDs and cloud storage services.