What Is a Hard Disk Drive?

Everything you need to know about computer hard drives

The hard disk drive is the main, and usually largest, 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" due to the fact that Microsoft Windows, by default, designates the "C" drive letter to the primary partition on the primary hard drive in a computer.

Illustration of a hard disc drive

While this isn't a technically correct term to use, it is still common. 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 type of drive but is almost always included with a hard drive purchase. 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 still have access to all the data when it's back on.

Inside the hard drive are sectors located on tracks, which are 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 common 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 basically a regular hard drive that's been put into 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 huge 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 true 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 change the partition letter to better remember which is which or any other reason. For example, while the main partition of a hard drive is normally called "C" and can't be changed, you can change an external hard drive's letter from "P" to "L" (or any other acceptable letter).

Something else that's really easy to do with a hard drive is check how much free space is left 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 really don't need to) defragment a solid state drive.

Since a hard drive is where all the data in a computer is actually 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. This is usually accomplished with a data destruction program.

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

Hard Disk Drive Troubleshooting

The hard drive in your computer is used over and over, each time you're doing something that involves reading or writing data to the disk. It's normal, then, to eventually run into a problem with the device.

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 own 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: