What is a hard disk very short answer?

What is a Hard Disk?

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 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. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data even when powered off.

Introducing the Hard Disk

Hard disk drives were introduced in the 1950s as data storage for mainframe computers. The first commercially available HDD was the IBM 350 RAMAC in 1956. The RAMAC HDD stored about 5 megabytes of data on fifty 24-inch platters. Over the decades, HDDs evolved to store more data at faster speeds and in smaller form factors. By the late 1980s, HDDs became the dominant secondary storage device for general purpose computers.

HDDs remain a common data storage solution in computers and servers today, though adoption of solid-state drives (SSDs) is increasing due to advantages such as faster read/write speeds, higher reliability, and smaller physical size. However, HDDs continue to offer far higher capacities at lower cost than SSDs. HDD capacity has grown from a few megabytes in the 1950s to multiple terabytes per drive today.

How a Hard Disk Works

A hard disk drive consists of one or more rigid platters and a read/write head assembly. Platters are made of a non-magnetic material, usually aluminum alloy, glass, or ceramic. Both sides of each platter are coated with a thin layer of magnetic material, typically 10-20 nanometers in depth, that stores the data.

The platters spin at very high speeds, typically 5400 to 15000 RPM for consumer HDDs and 10000 to 15000 RPM for enterprise models. The faster the platter spins, the faster data can be read from or written to it.

The read/write head assembly contains the read/write heads or transducers that move across the surfaces of each platter to access data. The assembly also contains the actuator arm which holds the heads and allows them to move in an arc across the platters.

Voice coils and servo mechanisms precisely position the heads over narrow tracks on the platters. Higher density HDDs cram more tracks onto each platter surface, allowing more data storage but requiring more precision from the servo mechanisms.

Reading and Writing Data

Each read/write head is a small electromagnetic coil suspended extremely close over the platter surface. To write data, an electrical current is sent through the coil, inducing a magnetic field on the platter’s surface and aligning magnetic particles into a pattern that encodes binary data. Reversing the polarity of the applied current will align the particles in the opposite direction, allowing 1s and 0s to be stored.

To read data, the head detects and converts changes in the magnetic flux into binary data. Platters are organized into concentric tracks which are further divided into sectors. By moving the actuating arm, the heads can seek to different tracks while the spinning platters bring sectors across the head for reading or writing.

Modern HDDs use tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) heads for greater data densities compared to older anisotropic magnetoresistive (AMR) and inductive head technologies.

Internal and External Hard Drives

Hard drives are classified as either internal or external. Internal HDDs are installed within a computer case and connected directly to the motherboard via standardized interfaces like SATA and M.2. External HDDs are connected externally via USB, Firewire, eSATA, or wirelessly.

Internal HDDs offer higher performance and are more commonly found in desktop PCs, servers, and laptops. External HDDs trade off some performance for portability and convenience. They can easily connect to different systems via cable or wirelessly.

Common Hard Disk Sizes

Hard disk sizes are measured in terms of physical dimensions as well as data storage capacity.

Common physical sizes for internal HDDs are:

– 3.5 inch – The most common desktop HDD size, holds up to ~18TB per drive. Requires external power from the PSU.

– 2.5 inch – Commonly used in laptops. Lower capacities than 3.5″ but can run off just USB power. Max ~5TB per drive.

– 1.8 inch – Very small HDDs mostly used in legacy portable devices, up to ~500GB.

– M.2 – A form factor that maxes out at ~4TB per drive currently. M.2 HDDs connect directly to the motherboard without cables.

Common external HDD sizes are:

– 3.5 inch – Requires external power adapter. Highest consumer capacities.

– 2.5 inch – Smaller but can run off just USB power. Up to ~5TB currently.

– 1.8 inch – Tiny HDDs for portable external storage. Lower capacities than 2.5″.

For data storage capacity, common HDD sizes include:

– 500GB to 2TB – Lower capacity consumer drives. Offer good value for typical users.

– 3TB to 6TB – Medium capacity consumer drives. Balance price and storage space.

– 8TB to 20TB – Highest capacity consumer drives. Ideal for advanced users.

– 10TB to 100TB+ – Enterprise and data center HDDs. Massive storage for servers and IT.

HDD Interface Types

Internal HDDs use standard interconnect interfaces to communicate with the computer:

– PATA – Legacy parallel ATA disks. Obsolete technology.

– SATA – Serial ATA. Most common modern internal HDD interface, SATA III runs at 6Gbps.

– SAS – Serial-Attached SCSI. Used in servers and RAID setups, runs at 12Gbps.

– M.2 – Designed for SSDs but some HDDs use it too. Extremely compact and fast with PCIe x4 lanes reaching nearly 4GB/s.

External portable HDDs use:

– USB – The most common interface for portable external disks with USB 2.0, 3.0, 3.1, or 3.2 interfaces at 480Mbps to 20Gbps.

– eSATA – External SATA allows speeds around 6Gbps in a portable form factor.

– Firewire – A legacy bus interface reaching 400 to 800Mbps.

– Thunderbolt – At up to 40Gbps, Thunderbolt offers the fastest single-drive bandwidth.

Wireless external hard drives communicate over WiFi or Bluetooth. While slower and less reliable than wired, they offer greater flexibility.

HDD Speed Measurements

Two key metrics determine HDD performance:

– Spindle speed – Rotations per minute (RPM) of the platter. Typical range is 5400 RPM (laptop HDDs) to 15000 RPM (high performance enterprise HDDs). Faster spin rates allow higher data transfer rates.

– Data transfer rate – The speed at which data travels between the platter and computer, measured in megabits per second (Mbps) or gigabits per second (Gbps). SATA III has a theoretical maximum of 6000 Mbps (6 Gbps).

Other HDD speed metrics:

– Average seek time – Time taken to position the read/write heads over a random data track on the platter. Typically between 8 and 15 milliseconds for modern HDDs.

– Cache – Amount of embedded memory, from 8MB to 256MB, that stores data most likely to be requested next. Larger cache improves performance.

– Interface bandwidth – The maximum theoretical bandwidth of the HDD interface, e.g. 600 MB/s for SATA III. The actual sustained transfer rate is lower.

HDD Interface Maximum bandwidth
SAS 1200 MB/s
USB 3.2 625 MB/s

HDD Capacity Measurements

Hard disk drive capacity is measured in gigabytes (GB) or terabytes (TB).

1 GB = 1000 megabytes (MB)

1 TB = 1000 GB

So a 500 GB HDD can store 500 billion bytes of data.

Actual formatted capacity is lower than advertised since some space is reserved for file system metadata, caching, and error correction. A 500 GB HDD may have only 465 GB usable in reality.

HDD Reliability and Lifespan

Hard disk drives have a typical annual failure rate of around 2-3% depending on the model and workload. Failure rates steadily increase after 2-3 years of use. Enterprise and NAS optimized drives designed for 24/7 operation tend to be more reliable than consumer PC drives.

Factors impacting HDD lifespan and reliability:

– Operating temperature – Higher temps increase failures. Cool data centers prolong life.

– Vibration and shock – Physical impacts damage internal components over time. SSDs are more durable.

– Duty cycles – Drives left powered on and active 24/7 wear out quicker than intermittently used ones.

– Recovery time – How long drives take to recover from vibration or shock events. Enterprise drives recover faster.

– Mean time between failures (MTBF) – Manufacturers rate enterprise drives for 1.2 million to 2.5 million hour MTBF. Consumer HDDs often range between 600,000 to 1.2 million hours.

– Daily workloads – Lightly used drives last longer than those subject to heavy activity with sustained reads and writes.

Overall HDD life expectancy has increased from around 5 years to around 10-15 years for current models.

Advantages of Hard Disk Drives

Compared to SSDs, hard disk drives offer:

– Lower cost per gigabyte – HDDs provide far more storage capacity at a given price point.

– Higher maximum capacities – Enterprise HDDs scale up to 100TB+. Best for high capacity storage needs.

– Proven reliability – HDD technology has been refined for decades making them a mature storage solution.

– Data recovery possibilities – Greater ability to recover failed HDD data using specialized tools.

For secondary storage, archival needs, network attached storage, and data centers, HDDs continue to offer advantages over SSDs when large amounts of storage space is required at a reasonable cost. The trade-off is lower performance.

Disadvantages of Hard Disk Drives

The downsides of using HDDs instead of SSDs include:

– Slower read/write speeds – HDDs have slower random I/O performance due to physical moving parts.

– Higher power consumption – Spinning platters and actuators require more electricity compared to SSDs.

– Increased noise – The mechanical components produce audible noise unlike silent SSDs.

– Lower resilience – Shock, vibration, and temperature extremes damage HDDs more easily than SSDs which have no moving parts.

– Larger physical size – Even 3.5″ HDD cases are bulky compared to compact M.2 SSDs which connect directly to a motherboard.

While HDDs continue to play an important role where large affordable storage is needed, SSDs are the better choice for general computing applications today. The focus has shifted to solid state drives for most consumer and business uses due to their inherent performance and reliability advantages.


Hard disk drives pioneered the secondary storage landscape and remain essential for modern data storage needs. HDD technology continues to evolve and sustain its dominance for mass capacity storage from home NAS devices to exabyte-scale data centers.

HDDs are electromechanical storage devices that store digital data on quickly rotating platters coated with magnetic material. Read/write heads on precision actuators access data by magnetizing and sensing platter surfaces. Though increasingly supplanted by SSDs for primary storage, HDDs deliver the raw capacity and value demanded by storage-intensive applications. Data center saying about HDDs: for the price of a pizza per TB.

While NAND flash drives like SSDs have faster access speeds, HDDs can still deliver reasonable real-world performance while offering much higher capacities. New technologies like shingled magnetic recording allow capacities to keep increasing on HDDs. Though eventually approaching practical limits, HDD technology promises to continue advancing incrementally for years to come before finally ceding the entire data storage market to solid state successors.