How Do Hard Drives Work?

How Do Hard Drives Work?

Last Updated on June 1, 2021 by Gabriel Goddy

How do hard drives work? You might ask. While this is a somewhat tricky question, we have an answer. If you read through this guide, you’ll have your question answered.

What is a hard drive?

A hard disk drive (sometimes abbreviated as a hard drive, HD, or HDD) is a non-volatile data storage device. It is usually installed internally in a computer, attached directly to the disk controller of the computer’s motherboard. It contains one or more platters, housed inside of an air-sealed casing. Data is written to the platters using a magnetic head, which moves rapidly over them as they spin.

What are the parts in a hard drive?

A hard drive has only a few basic parts. There are one or more shiny silver platters where information is stored magnetically, there’s an arm mechanism that moves a tiny magnet called a read-write head back and forth over the platters to record or store information, and there’s an electronic circuit to control everything and act as a link between the hard drive and the rest of your computer.

  • The actuator moves the read-write arm. In older hard drives, the actuators were stepper motors. In most modern hard drives, voice coils are used instead. As their name suggests, these are simple electromagnets, working rather like the moving coils that make sounds in loudspeakers. They position the read-write arm more quickly, precisely, and reliably than stepper motors and are less sensitive to problems such as temperature variations.
  • Read-write arm swings read-write head back and forth across the platter.
  • The central spindle allows the platter to rotate at high speed.
  • Magnetic platter stores information in binary form.
  • Plug connections link the hard drive to the circuit board in a personal computer.
  • The read-write head is a tiny magnet on the end of the read-write arm.
  • The Circuit board on the underside controls the flow of data to and from the platter.
  • Flexible connector carries data from the circuit board to read-write head and platter.
  • A small spindle allows the read-write arm to swing across the platter.

Two types of drives

A hard drive can be internal or external.

Internal means a hard drive is located inside a computing device and directly connects to the motherboard, but it’s not always upgradable. For instance, a desktop side can be easily removed to disconnect the old drive and connect the new drive. It’s a quick, simple upgrade.

On laptops, however, the upgrade process may not be quite so simple. Typically there is a door along the bottom providing access to the drive. Other laptops, like Apple’s MacBooks, don’t have removable storage. Read the specifications on the manufacturer’s websites on how to change a laptop’s drive properly.

External means a hard drive is located outside the PC and typically connects through a USB or Thunderbolt cable. This option is typically slower due to the connection, but it can also be detached from the parent PC without any major issues.

Besides internal and external, a hard drive can be a hard disk drive (HDD) or a solid-state drive (SSD). There’s a huge difference between the two that we explain in a separate article, SSD versus HDD. However, here are the shorter explanations:

HDD: Hard drive disks use a spinning magnetic disk that holds information inscribed in very tiny tracks — a bit like a record player. This requires moving parts, specifically heads to read and write data to the disk as needed and propulsion to spin the disk. It’s a simple method, making HDDs very inexpensive to purchase, especially when creating extensive storage setups.

Hard disk drives are also known as hard disk drives because the platters they employ are disc-shaped. Each platter is coated in a special thin film of magnetic material whose field can be flipped when an external magnetic field is applied.

SSD: There are no moving parts in SSDs. Instead, these drives use semiconductors that store information by changing the electrical state of very tiny capacitors. They are much faster than HDDs and can store information more easily without the risk of parts wearing out. SSDs are why modern PCs boot up so fast.

Important Hard Drive Qualities

Speed: The speed of a hard drive depends on how fast it can read or write data. The connection to the PC also factors in. A poor connection can bottleneck the data flow and, ultimately, impact the machine’s performance. For mechanical hard drives, the spin speed is also essential: 7,200RPM drives, for example, are faster than 5,400RPM drives. Both are far slower than SSDs.

Physical security: Hard drives need to be able to resist the occasional jolts and bumps. That’s what physical security is all about — your device’s resistance to damage that could result in data loss. Physical security is primarily about durability, and it’s an important consideration for both internally-mounted and external hard drives.

Environmental factors, such as extreme heat or cold, are an essential consideration. Also, some manufactures include features that help prevent hacking or discourage theft.

Connections: Choose a hard drive with ports that fit your computer, like PCI Express, Thunderbolt, USB, or SATA. Check out our guide to know more about SATA. Make sure you know your computer connection types.

Different hard drives affect speed differently. An external solid-state drive connected to an old port will significantly restrict data flow by 60 megabytes per second.

How a hard drive works

In your computer’s hard drive, there aren’t really any iron nails. There’s just a large shiny, circular “plate” of magnetic material called a platter, divided into billions of tiny areas. Each one of those areas can be independently magnetized (to store a 1) or demagnetized (to store a 0).

Magnetism is used in computer storage because it goes on to store information even when the power is switched off. If you magnetize a nail, it stays magnetized until you demagnetize it. In much the same way, the computerized information (or data) stored in your PC hard drive or iPod stays there even when you switch the power off.

The platters are the most important parts of a hard drive. As the name suggests, they are disks made from a hard material such as glass, ceramic, or aluminum, which is coated with a thin layer of metal that can be magnetized or demagnetized.

A small hard drive typically has only one platter, but each side of it has a magnetic coating. Bigger drives have a series of platters stacked on a central spindle, with a small gap in between them. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them.

There are two read-write heads for each platter, one to read the top surface and one to read the bottom, so a hard drive that has five platters (say) would need ten separate read-write heads. The read-write heads are mounted on an electrically controlled arm that moves from the center of the drive to the outer edge and back again. To reduce wear and tear, they don’t actually touch the platter: there’s a layer of fluid or air between the head and the platter surface.

What are the sizes of hard drives?

The hard drive is often capable of storing more data than any other drive, but its size can vary depending on the type of drive and its age. Older hard drives had a storage size of several hundred MB (megabytes) to several GB (gigabytes). Newer hard drives have a storage size of several hundred gigabytes to several TB (terabytes). Each year, new and improved technology allows for increasing hard drive storage sizes.

How is data read and stored on a hard drive?

Data sent to and read from the hard drive is interpreted by the disk controller. This device tells the hard drive what to do and how to move its components. When the operating system needs to read or write information, it examines the hard drive’s FAT (File Allocation Table) to determine file location and available write areas.

Once that is determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information.

All information saved and stored on a traditional hard drive, like the above example, is done magnetically. After completing the above steps, if the computer needs to read information from the hard drive, it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0, and the other is 1.

Reading this as binary data, the computer can understand what the data is on the platter. For the computer to write information to the platter, the read/write head aligns the magnetic polarities, writing 0’s and 1’s that can be read later.


Everyone uses computers, but surprisingly few understand how computer devices work, how a hard drive work, and their most essential components. Such knowledge is helpful to anyone who wants to maintain control over their mission-critical data or recover it after a data loss.

While hard drives are built to last for decades, problems can really arise at the most unexpected times. Your hard drive encountering problems can lead to you losing hours and hours of hard work.

Saving a backup is always something tech experts will recommend to you. Even if you mastered how a hard disk drive works, everything you learned will be put to waste if you forget about learning how to backup files.