What does a hard drive do?

A hard drive is one of the most important components in modern computers. It is the storage device that allows a computer to store and access data quickly and efficiently. At its core, a hard drive’s purpose is to store and retrieve digital information. This digital information could include everything from your operating system, applications, documents, music, movies and more.

What is a hard drive?

A hard drive is a non-volatile storage device, meaning it retains data even when powered off. Hard drives use magnetic storage to store and access data on one or more rapidly spinning disks coated with magnetic material. The data itself is recorded on these disks in the form of bits – 1s and 0s. A read/write head floats just above the disk reading and writing data as the disk spins.

The most common type of hard drive is the HDD or hard disk drive. HDDs use mechanical parts like the spinning platter and read/write head to access data. Another type of hard drive is the solid state drive or SSD. SSDs have no moving parts and instead use flash memory chips to store data. Compared to HDDs, SSDs are typically faster, lighter, and more reliable since they have no mechanical parts.

What does a hard drive do?

The primary purpose of a hard drive is as long-term storage for a computer’s data. Here are some of the key jobs a hard drive performs:

  • Operating system storage – The hard drive stores the operating system files like Windows, MacOS or Linux. These system files allow your computer to startup and function.
  • Program and application storage – Software, apps, and programs are installed and run from the hard drive.
  • Document storage – Word docs, excel sheets, PDFs and more are stored on the hard drive.
  • Media storage – Photos, music, videos and other media files are stored on the hard drive.
  • File storage/retrieval – The hard drive allows for long term storage, organization and retrieval of computer files.
  • Quick access to data – Data can be rapidly accessed from the hard drive much faster than external storage like DVDs or USB drives.

In essence, the hard drive is the long-term memory of the computer. While RAM provides fast short-term storage, the hard drive allows for large amounts of data to be permanently stored. Everything from personal files to software lives on the hard drive when not in current use.

How does a hard drive store data?

Hard drives store data on rapidly spinning disks inside the hard drive enclosure. These disks are coated in a thin magnetic film consisting of microscopic magnetic particles. The data itself is recorded on these particles.

The disks spin at incredibly fast speeds, usually 5400 RPM to 7200 RPM for consumer hard drives. As the disks spin, the read/write head floats just above the surface reading and writing data. The faster the disks spin, the faster data can be accessed.

Data is stored in binary code – strings of 1s and 0s. These 1s and 0s align with the polarity of the magnetic particles. A 1 may be positive and a 0 negative for example. The read/write head is able to detect these magnetic polarities and convert them back into binary data.

The surface of each disk is divided into concentric circles called tracks. Tracks are further divided into sectors which represent the smallest unit of data storage on a hard drive. A single bit is stored in each magnetic particle within each sector. By reading millions of bits in sequence, the drive can store and retrieve usable data.


On a physical hard disk, tracks that are the same distance from the center of the disk make up a cylinder. Reading and writing data within a cylinder is faster than moving the read/write heads across the discs to different cylinders. Data is therefore optimally organized into cylinders for quickest access.

Hard Drive Interface

The hard drive interface is how the drive communicates with the computer system. Here are some common hard drive interface types:

  • SATA – Serial ATA is the most popular interface for both HDDs and SSDs in desktop and laptop computers. SATA interfaces transfer data serially and use thin cables.
  • SAS – Serial Attached SCSI is a high speed serial interface designed for enterprise and server drives. SAS is similar to SATA but built for heavy workloads.
  • USB – Portable external hard drives typically use USB interfaces which allow the drive to be connected via USB cable.
  • NVMe – NVMe or Non-Volatile Memory Express is a super fast PCIe interface for SSDs to connect directly with the system’s motherboard.

Hard Drive Performance

Some key specifications that determine hard drive performance are:

  • Spindle speed (RPM) – Faster spindle speeds allow data to be accessed quicker.
  • Cache size – Larger cache buffers improve read/write speeds.
  • Average seek time – Seek time is the delay for the head to move to a track.
  • Interface type – Newer interfaces like SATA, SAS and NVMe are much faster.
  • Form factor – 2.5″ vs 3.5″ drives – generally determines max speeds.

For average computer users, some general guidelines for performance are:

Use Case Recommended Hard Drive
Basic desktop usage 5400 RPM 3.5″ SATA HDD
Gaming and creative work 7200 RPM 3.5″ SATA HDD or SATA SSD
Performance intensive work NVMe PCIe SSD

As a general rule, SSDs will provide much faster access speeds than HDDs. However, HDDs have higher capacities for less money.

Hard Drive Capacity

Hard drive capacity measures the total data storage space available on a drive. Capacity is measured in gigabytes (GB) or terabytes (TB).

1 TB = 1000 GB

Some examples of hard drive capacities for consumer drives:


– 1-2 TB for basic desktops
– 3-6 TB for standard use
– 8-10+ TB for photo/video storage


– 128-256 GB for boot drives
– 250GB-1TB for faster performance
– 2TB+ for heavy usage

Enterprise and server hard drives may have even higher capacities reaching into the petabytes (PB).

Choosing the right capacity depends on your budget and storage needs. A good rule of thumb is to choose the largest capacity you can reasonably afford.

How does the computer access data from the hard drive?

When the computer needs to access data from the hard drive, a process takes place between the system’s processor, cache, RAM and hard drive controller known as the computer’s storage hierarchy:

  1. The processor first looks for the data in the cache memory. Cache memory is super fast memory embedded directly on the CPU.
  2. If the data is not in cache, the CPU then looks in system RAM. RAM provides fast temporary storage for currently used data.
  3. If the data is not in RAM, the processor sends a request via the SATA bus to the hard drive controller.
  4. The hard drive controller retrieves the data from the physical discs by moving the read/write head to the proper track and sector.
  5. The data is transferred via the SATA interface back to RAM for fast access by the processor.

By storing frequently accessed data in cache and RAM, the processor can access it much faster than having to read it directly from the hard drive each time it’s needed. But hard drives provide the massive long-term storage space to keep data persistently stored when the computer is off.

Are hard drives fragile?

Generally speaking, hard drives are fairly fragile devices and are susceptible to damage from shock, vibration, power outages, temperature extremes, demagnetization and more. However, when handled properly and kept in stable environments, today’s hard drives can reliably operate for several years.

HDDs are more mechanically fragile than SSDs since they use moving parts like the head actuator arm. Dropping a powered hard drive can cause the head to hit the disk at high speed and damage the platters. SSDs have no moving parts so are not vulnerable to head crashes or mechanical wear.

To protect the drive, laptops and desktop computers have shock mounting systems and accelerometers that detect falls and lock the head in place. Portable external hard drives are more prone to damage from drops or impacts during transit so should be handled with care.

Magnets can erase data by re-aligning the magnetic particles on the disk. Therefore drives should be kept away from magnets and high electromagnetic fields. Excessive heat can also damage drive components over time leading to potential data loss.

How are hard drives protected from data loss?

While total hard drive failure is always a risk, hard drives are designed with redundancy mechanisms to protect and recover data if parts of the drive fail. Two key technologies are:

  • SMART monitoring – SMART (Self-Monitoring Analysis and Reporting Technology) tracks drive attributes like temperature, errors, bad sectors, spin speeds and more. It alerts you to impending drive issues.
  • RAID – RAID (Redundant Array of Independent Disks) allows drives to be mirrored or striped together for redundancy and better performance.

Recovering lost data from a damaged hard drive may require specialized data recovery services to repair the drive and read platters in a dust-free clean room environment.

Main Components of a Hard Drive

While there are small internal design differences between HDDs and SSDs, in general hard drives consist of these core components:

  • Platters – The disks that data is recorded onto. Platters are made of non-magnetic material like aluminum or glass and are coated with a thin magnetic film for data storage.
  • Spindle – The spindle is the center rod the platters rotate around. Its high-precision bearings help the platters spin at fast, stable speeds.
  • Read/Write Head – Located at the end of the actuator arm, the read/write head flies just above the disk surface, reading and writing data.
  • Actuator arm – Moves the read/write head across the platters during drive operations.
  • Motor – Provides the rotational force to spin the disks at high speeds.
  • Firmware – The hard drive’s onboard firmware controls drive functions like read/write commands, caching, self-monitoring and data translation between binary and computer readable data.
  • Interface – The hard drive’s interface like SATA or SAS connects it to the computer’s motherboard, CPU and RAM.
  • Casing – Protects the internal components from damage and contamination.

By coordinating all these parts, hard drives can reliably store vast amounts of data and provide quick access times for reading and writing data.


A computer hard drive is the primary long-term data storage device within computers and servers. Using magnetic storage technologies, hard drives are able to persistently store and retrieve digital data including the operating system, software, documents, media files and more. They provide non-volatile storage, meaning data is retained even when the drive is powered off.

Key innovations like integrated drive electronics, smaller form factors, higher RPM spindle speeds and improved interfaces have steadily increased hard drive capacities while reducing physical size and access times. Hard drives continue to be a critical component of computer systems alongside temporary storage technologies like RAM and flash-based solid state drives.