What Is a Hard Disk Drive?

Everything you need to know about computer hard drives

The hard disk drive is the main, and usually most substantial, data storage hardware device in a computer. The operating system, software titles, and most other files are stored in the hard disk drive.

The hard drive is sometimes referred to as the «C drive» because Microsoft Windows, by default, designates the «C» drive letter to the primary partition on the primary hard drive in a computer.

While this isn’t a technically correct term to use, it is still prevalent. For example, some computers have multiple drive letters (e.g., C, D, and E) representing areas across one or more hard drives. The hard disk drive also goes by the name HDD (its abbreviation), hard drive, hard disk, magnetic hard drive, mechanical hard drive, fixed drive, fixed disk, and fixed disk drive.

Regardless of what it’s called, the primary hard drive typically contains the root folder of the operating system used.

Hard Disk Drive Physical Description

A hard drive is usually the size of a paperback book, but much heavier.

The sides of the hard drive have pre-drilled, threaded holes for easy mounting in the 3.5-inch drive bay in the computer case. Mounting is also possible in a smaller 2.5-inch drive and larger 5.25-inch drive bay with an adapter. The hard drive is mounted, so the end with the connections faces inside the computer. Laptops often use a 2.5-inch hard drive or SSD.

The back end of the hard drive contains a port for a cable that connects to the motherboard. The type of cable used (SATA or PATA) depends on the kind of drive, but most hard drive purchases include them. Also, here is a connection for power from the power supply.

Most hard drives also have jumper settings on the back end that define how the motherboard is to recognize the drive when more than one is present. These settings vary from drive to drive, so check with your hard drive manufacturer for details.

How a Hard Drive Works

Unlike volatile storage like RAM, a hard drive keeps a hold of its data even when powered off. This is why you can restart a computer, which powers down the HDD, but retain access to all the data when it’s back on.

Inside the hard drive are sectors located on tracks, stored on rotating platters. These platters have magnetic heads that move with an actuator arm to read and write data to the drive.

Kinds of Hard Drives

The computer hard drive isn’t the only kind of hard drive, and SATA and PATA aren’t the only ways they can connect to a computer. What’s more is that there are many different sizes of hard drives, some very small and others rather large.

For example, the standard flash drive has a hard drive, too, but it doesn’t spin like a traditional hard drive. Flash drives are sometimes referred to as solid-state drives and connect to the computer through USB. There are also hybrids called SSHDs.

Another USB hard drive is the external hard drive, which is a regular hard drive in its own case so that it’s safe to exist outside the computer case. They usually interface with the computer over USB, but some use FireWire or eSATA.

An external enclosure is a housing for an internal hard drive. You can use one if you want to «convert» an internal hard drive into an external one to make your own external hard drive. They, too, use USB, FireWire, and so forth.

Storage Capacity

The hard disk drive capacity is a significant factor in determining whether someone will buy a particular device like a laptop or phone. If the storage capacity is rather small, it means it will fill up with files faster, whereas a drive that has lots and lots of storage can handle much more data.

Choosing a hard drive based on how much storage it can retain is really up to opinion and circumstance. If you need a tablet, for example, that can hold lots of videos, you’ll want to be sure to get the 64 GB one instead of the 8 GB one.

The same is valid for computer hard drives. Are you one to store lots of HD videos or pictures, or are most of your files backed up online? An offline, at-home storage preference might drive you to buy an internal or external hard drive that supports 4 TB versus a 500 GB one.

Common Hard Disk Drive Tasks

One simple task that you can do with a hard drive is to change the partition letter to remember better which is which or any other reason. For example, while the primary partition of a hard drive is customarily called «C» and can’t be changed, you can switch an external hard drive’s letter from «P» to «L» (or any other acceptable letter).

Something else that’s easy to do with a hard drive is to check how much free space remains on it. This is especially important if you’re getting low disk space messages but is equally critical for maintaining a smooth system. You can uninstall programs you don’t want or ones that are too large, and delete files, or copy them elsewhere if you’re running low on hard drive space.

You need to format the drive or partition the drive into sections before you can install an operating system or store files. Upon installing the OS for the first time is usually when a new hard drive is formatted and given a file system. Otherwise, a disk partition tool is a common way to manipulate the drive in this way.

When you’re dealing with a fragmented hard drive, free defrag tools ​are available that can help reduce the fragmentation. Defragging a hard drive can sometimes make your computer run faster.

You should not (and don’t need to) defragment a solid-state drive.

Since a hard drive is where all the data in a computer is stored, it’s a common task to want to securely erase the data from the drive, like before selling the hardware or reinstalling a new operating system. You can usually accomplish this with a data destruction program.

On the opposite end, you can protect your hard drive data with a disk encryption program. Only after you enter a specific password will the data be decrypted and readable/usable.

Hard Disk Drive Troubleshooting

Your computer uses the hard drive over and over, each time you’re doing something that involves reading or writing data to the disk. It’s common to run into a problem with the device eventually.

One of the most common issues is a hard drive that’s making noise, and the best first step in troubleshooting a hard drive malfunction of any kind is to run a hard drive test.

Windows includes a built-in tool called chkdsk that helps identify and maybe even correct various hard drive errors. You can run the graphical version of this tool in most versions of Windows.

Lots of free programs can test a hard drive ​for issues that might ultimately lead to you needing to replace the drive. Some of them can also measure performance like the seek time.

Buying a New Hard Drive

A few of the most popular hard drive manufacturers include Seagate, Western Digital, Hitachi, and Toshiba.

You can usually buy these brands of hard drives, and ones from other manufacturers, in stores and online, like through the company’s sites as well as sites like Amazon.

If you’re not sure where to start, see our updated lists of the best hard drives of all types, including:

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.

Internal hard disks reside in a drive bay, connected to the motherboard using an ATA, SCSI, or SATA cable. They are powered by a connection to the computer’s PSU (power supply unit).

Examples of data stored on a computer’s hard drive include the operating system, installed software, and the user’s personal files.

Why does a computer need a hard drive?

A computer requires an operating system to allow users to interact with and use it. The operating system interprets keyboard and mouse movements and allows for the use of software, like an Internet browser, word processor, and video games. To install a computer operating system, a hard drive (or another storage device) is required. The storage device provides the storage medium where the operating system is installed and stored.

A hard drive is also required for the installation of any programs or other files you want to keep on your computer. When downloading files to your computer, they are permanently stored on your hard drive or another storage medium until they are moved or uninstalled.

Can a computer work without a hard drive?

Without a hard drive, a computer can turn on and POST. Depending on how the BIOS is configured, other bootable devices in the boot sequence are also checked for the necessary boot files. For example, if the USB device is listed in your BIOS boot sequence, you can boot from a bootable USB flash drive in a computer without a hard drive.

Examples of bootable flash drives include a Microsoft Windows installation disk, GParted Live, Ubuntu Live, or UBCD. Some computers also support booting over a network with a PXE (preboot execution environment).

Hard drives in modern computers

Modern computers often use an SSD (solid-state drive) as the primary storage device, instead of an HDD. HDDs are slower than SSDs when reading and writing data, but offer greater storage capacity for the price.

Although an HDD may still be used as a computer’s primary storage, it’s common for it to be installed as a secondary disk drive. For example, the primary SSD may contain the operating system and installed software, and a secondary HDD may be used to store documents, downloads, and audio or video files.

New computer users may confuse RAM (memory) with their disk drive. Unlike an HDD or SSD, RAM is a «volatile» data storage device, meaning it can store data only when the computer is powered on. See our memory definition for a comparison between memory and disk storage.

Hard drive components

As shown in the picture above, the desktop hard drive consists of the following components: the head actuator, read/write actuator arm, read/write head, spindle, and platter. On the back of a hard drive is a circuit board called the disk controller or interface board. This circuit is what allows the hard drive to communicate with the computer.

How is a hard drive connected to a computer?

An internal hard drive connects to the computer using two means: a data cable (IDE, SATA, or SCSI) to the motherboard and a power cable to the power supply.

Where is the hard drive found in a computer?

All primary computer hard drives are found inside a computer case and are attached to the computer motherboard using an ATA, SCSI, or SATA cable. Hard drives are powered by a connection to the PSU (power supply unit).

Some portable and desktop computers may have newer flash drives that connect directly to the PCIe interface or another interface and not use a cable.

What is stored on a hard drive?

A hard drive can store any data, including pictures, music, videos, text documents, and any files created or downloaded. Also, hard drives store files for the operating system and software programs that run on the computer.

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.

If you are trying to find the physical dimensions of a hard drive, their physical sizes are either 3.5″ for desktop computers or 2.5″ for laptops. SSDs range from 1.8″ to 5.25″.

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.

External and internal hard drives

Although most hard drives are internal, there are also stand-alone devices called external hard drives or portable hard drives that backup data on computers and expand the available space. External drives are often stored in an enclosure that helps protect the drive and allows it to interface with the computer, usually over USB, eSATA, or FireWire. An excellent example of an external backup device that supports multiple hard drives is the Drobo.

External hard drives come in many shapes and sizes. Some are large, about the size of a book, while others are about the size of a large smartphone. External hard drives can be very useful since they usually offer more space than a jump drive and are still portable. The picture is an example of a laptop hard disk drive enclosure from Adaptec. With this enclosure, the user installs a laptop hard drive of any storage capacity into the enclosure and connect it via USB port to the computer.

HDD being replaced by SSD

SSDs (solid-state drives) have started to replace HDDs (hard disk drives) because of the distinct performance advantages they have over HDD, including faster access times and lower latency. While SSDs are becoming popular, HDDs continue to be used in many desktop computers largely due to the value per dollar that HDDs offer over SSDs. However, more and more laptops are beginning to utilize SSD over HDD, helping to improve the reliability and stability of laptops.

History of the hard drive

The first hard drive was introduced to the market by IBM on September 13, 1956. The hard drive was first used in the RAMAC 305 system, with a storage capacity of 5 MB and a cost of about $50,000 ($10,000 per megabyte). The hard drive was built-in to the computer and was not removable.

In 1963, IBM developed the first removable hard drive, having a 2.6 MB storage capacity.

The first hard drive to have a storage capacity of one gigabyte was also developed by IBM in 1980. It weighed 550-pounds and cost $40,000.

1983 marked the introduction of the first 3.5-inch size hard drive, developed by Rodime. It had a storage capacity of 10 MB.

Seagate was the first company to introduce a 7200 RPM hard drive in 1992. Seagate also introduced the first 10,000 RPM hard drive in 1996 and the first 15,000 RPM hard drive in 2000.

Should I say «hard disk drive» or «hard drive»?

Both «hard disk drive» and «hard drive» are correct and mean the same thing. However, we recommend using the term «hard drive» in your writing or when describing a hard drive. The term «hard drive» helps to distinguish it from an SSD (solid-state drive), which contains no platters, disk-shaped components, or moving parts.