Date First Published: 3rd July 2022
Topic: Computer Systems
Subtopic: Computer Hardware
Article Type: Computer Terms & Definitions
Difficulty: MediumDifficulty Level: 4/10
Learn more about what a hard disk drive is in this article.
A hard disk drive, also known as a hard disk, hard drive, or HDD, is a non-volatile storage device used to store data on a computer. Everything on a computer, including the operating system, documents, images, videos, and software programs is stored on the HDD. Even though the terms 'hard disk drive', 'hard disk', and 'hard drive' refer to the same thing, the most accurate term is 'hard disk drive' as the hard disk is found in the hard disk drive.
SSDs (solid-state drives) have become an alternative to HDDs. Even though HDDs are still widely used in laptops and desktop computers, SSDs have started to replace HDDs due to faster read/write speeds, less power consumption, less latency, and no moving parts.
Because HDDs are non-volatile storage devices, they store data even when the computer is turned off, unlike RAM which loses the stored data when the power is turned off.
Hard disk drives are located in the drive bay and are connected to the motherboard through a SATA, ATA, or SCSI cable inside the computer case. They are usually located at the front and are secured by screws. Therefore, if any issues occur with the hard disk drive, it can be separately replaced rather than having to buy a whole new computer.
In laptop computers, hard disk drives can be located at different positions around the laptop, depending on the model. It can be located on the side, underneath the laptop, or directly in front of the unit.
Mobile devices do not use hard disk drives. Instead, they use NAND flash memory for their non-volatile storage in combination with RAM to quickly access apps and programs. Mobile devices generally store data in the same way as an SSD does.
In Microsoft Windows, the hard disk drive is referred to as the 'C' drive because it is assigned to the hard disk partition, which contains the operating system and other files. Originally, drives A and B were reserved for floppy disks. The letter 'C' was assigned to the hard disk partition as it was the first available letter. The letter-naming scheme for these drives goes back to the early days of DOS (Disk operating system) machines.
Without a hard disk drive, a computer can still work. Computers can boot through a USB drive, CD, or DVD. If a computer is booted without any hard disk drive, the computer will still turn on and display the BIOS screen, but the computer would not load as there would be no operating system, making it useless. An error message may display saying that the boot device was not found.
HDDs consist of multiple disks that are made up of magnetic material called platters. HDDs magnetise and demagnetise billions of separate areas on the platter. Since computers store data in binary, which consists of 0s and 1s, a magnetised area represents a 1 and a demagnetised area represents a 0.
A write head records data that writes bits onto the disk and a read head reads data from the disk. These write and read heads can be found at the end of a tapered metal component, known as an actuator arm. They look similar to a turntable, but the hard disk spins much faster than a record. In fact, it spins hundreds of times faster. Also, HDDs read data digitally with 1s and 0s, whilst record players read an analogue signal.
The back of a hard disk drive consists of a circuit board with the disk controller or interface board, which allows the hard disk drive to communicate with the computer.
The speed of hard disks is measured in revolutions per minute (RPM). This measures the rotational speed of hard disk platters and has the greatest impact on the overall speed it can read and write data. A higher RPM will lead to data being accessed faster and a lower RPM will lead to data being accessed slower, causing the computer to spend much more time waiting for the hard disk drive. Common RPM rates are between 1200 and 15,000 RPM. The RPM of a hard disk drive cannot be increased and the only way to increase it is to replace it.
Since SSDs have no movable parts, they have no RPM.
Hard disk drives can store large amounts of data. In desktop computers, terabyte (1000 gigabyte) HDDs are common and it is possible to get one for less than £50. The storage capacity of a hard disk drive depends on the type of HDD. Some HDDs can store up to several terabytes of data, but these are very expensive. Therefore, it is only recommended to buy what you need.
In 1953, engineers at IBM recognised the need for a form of data storage after considering technologies, such as wire matrices, rod arrays, drums, drum arrays, etc. They wanted to find a way of providing random access to large amounts of data at a low cost. The first hard disk drive developed was the size of a fridge that could store up to 3.75 MB of data. It was shipped in June 1956 as part of the IBM 350 RAMAC system.
The storage capacity of hard disk drives has significantly increased over time. When hard disk drives were first available for personal computers, they only provided a 5 MB capacity. During the mid-1990s, the common hard disk drive for a personal computer had a capacity of around 1 GB. As of August 2020, hard disk drives have a capacity of several terabytes. In 2021, Western Digital Technologies revealed two 20-terabyte HDDs.
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