What Is a Hard Drive? - Types, Function & Definition

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Paul Zandbergen

Paul has a PhD from the University of British Columbia and has taught Geographic Information Systems, statistics and computer programming for 15 years.

There are several different types of hard drives for storing software and data files: hard disk drives, solid-state drives and external drives. Learn about how they work and how to select the right drive for a particular computer system.

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You may have experienced this scenario: you turn on your computer and you get a black screen with the message 'Imminent Hard Disk Failure'. Or you've dropped your laptop on the floor, your screen goes black and your computer won't turn back on again. More often than not the problem is your hard drive. If your hard drive has crashed, you may have lost all your documents, photos, music, etc. It can be one of the more frustrating experiences for any computer user. Read on to learn how your hard drive works, and some simple steps you can take to avoid losing all your files if it crashes.


The hard drive of a computer is a device that stores all the software installed on a computer as well as all the data files created and used by this software. This includes any documents you have created and downloaded, such as photos and music. The hard drive is a form of permanent storage, contrary to temporary memory such as Random-access memory (RAM). This means that when you turn off the computer the files remain safely stored on the drive so you can use them again the next time you start your computer.

There are two general types of hard drives: hard disk drives (HDD), which use one or more rotating discs and rely on magnetic storage, and solid-state drives (SDD), which have no moving mechanical parts but use flash memory like the kind found in USB flash drives. If you have a regular desktop computer, you most likely have a hard disk drive. Solid-state drives are more typical for high-end, expensive laptops.

Every computer has at least one internal hard drive to store software and data. If you are using a Windows operating system this drive is typically called the C drive. If you are using a Mac it is just called the hard drive. If you need extra storage capacity you can install additional internal hard drives or connect separate external hard drives.

Hard disk drives (HDD)

Hard disk drives have been the dominant type of storage since the early days of computers. A hard disk drive consists of a rigid disc made with non-magnetic material which is coated with a thin layer of magnetic material. Data is stored by magnetizing this thin film. The disk spins at high speed and a magnetic head mounted on a moving arm is used to read and write data. A typical hard disk drive operates at a speed of 7,200 rpm (rotations per minute), so you will often see this number as part of the technical specifications of a computer. The spinning of the disk is also the source of the humming noise of a computer, although most modern hard disk drives are fairly quiet.

Photograph of a hard drive - the disk and moving arm are clearly visible
hard disk drive

In general, hard disk drives are very robust and can be used for many years without problems. However, hard disk drives can fail and one of the most common reasons is a head crash. This occurs when the magnetic head scratches the magnetic film. This typically happens as a result of a physical shock, like dropping a computer while it is on. When your hard drives experiences mechanical failure you can often hear a grinding or scratching sound. Such as crash results in data loss since the magnetic film gets damaged. It is therefore always a good idea to have a backup copy of the important files on your hard drive.

Solid-state drives (SDD)

Solid-state drives (SDD) are a relatively new alternative to more traditional hard disk drives. Solid-state drives do not have moving parts, and data is stored electrically instead of magnetically. Most solid-state drives use flash memory, which is also used in memory cards for digital cameras and USB flash drives. Since there are no moving parts, solid-state drives are much less vulnerable to damage from physical shock. The major downside of solid-state drives is that they are a lot more expensive than hard disk drives, although prices are gradually coming down.

Photograph of a solid-state drive - there are no moving parts
solid-state drive

Despite the cost, solid-state drives are quickly becoming the preferred type of hard drive for certain types of computers because they are very damage proof and smaller than regular hard disk drives. For example, the MacBook Air now comes standard with a solid-state drive using flash memory.

Capacity and performance

The most important characteristic of a hard drive is how much data it can store, referred to as the storage capacity. A typical internal hard drive for a new desktop computer or laptop has a storage capacity of several hundred gigabytes (GB) up to 1 terabyte (TB). How large is a terabyte? Consider that a typical song in MP3 format is in the order of 5 to 10 MB. You could store approximately 150,000 songs on a 1 TB drive.

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