https://www.techtarget.com/searchstorage/definition/hard-disk-drive
A computer hard disk drive (HDD) is a non-volatile data storage device. Non-volatile refers to storage devices that maintain stored data when turned off. All computers need a storage device, and HDDs are just one type.
HDDs are usually installed in desktop computers, mobile devices, consumer electronics and enterprise storage arrays in data centers. They store operating systems (OSes), software programs and other files using magnetic disks.
More specifically, hard disk drives control the reading and writing of the hard disk that provides data storage. HDDs are used either as the primary or secondary storage device in a computer. They are commonly found in the drive bay and are connected to the motherboard via an Advanced Technology Attachment, Serial ATA, parallel ATA or Small Computer System Interface (SCSI) cable, among other formats. The HDD is also connected to a power supply unit and can retain stored data while powered down.
A hard disk drive -- often shortened to hard drive -- and hard disk are not the same things, but they are packaged as a unit and either term can refer to the whole unit.
Storage devices such as hard disks are needed to install OSes, programs and additional storage devices, and to save documents. Without devices like HDDs that retain data after they've been turned off, computer users wouldn't be able to store programs, files or documents on their computers. This is why every computer needs at least one storage device to permanently hold data as long as it's needed.
HDDs are for long-term storage. Random access memory, on the other hand, only stores data for immediate use until a computer is either shut down or restarted. RAM is also referred to as volatile memory. Specific file types, such as music, pictures or videos, are meant for long-term storage, making storage devices necessary.
Most basic hard drives consist of several disk platters, which are circular disks made of either aluminum, glass or ceramic with a thin magnetic coating. The platters are positioned around a spindle inside a sealed chamber.
A motor connected to the spindle spins the platters at up to 15,000 rotations per minute. As the platters spin, a second motor controls the position of the read and write heads that record and read information to and from tracks on the platters using a magnetic head
Some of the most common storage drive capacities include the following:
The highest capacity HDD available is 36 TB. However, an HDD has less usable space than advertised because the OS, file system structures and some data redundancy procedures use a portion of the HDD. Enterprise use of disk drives, such as for large-scale file collections, can require anywhere from a few terabytes to the maximum number of terabytes on an HDD. Home office users often require less than a terabyte.
Hard disk drive components include the spindle, disk platter, actuator, actuator arm and read/write head. Even though the term hard disk drive is used to refer to the unit as a whole, the term hard disk is the set of stacked disks. This is the part of the HDD that stores and provides access to data on an electromagnetically charged surface.
The HDD form factor refers to the physical size or geometry of the data storage device. HDD form factors follow a set of industry standards that govern their length, width and height, as well as the position and orientation of the host interface connector. Having an industry-standard form factor helps determine a common compatibility with different computing devices.
The most common form factors for HDDs in enterprise systems are 2.5-inch and 3.5-inch -- also known as small form factor and large form factor. The 2.5-inch and 3.5-inch measurements represent the approximate diameter of the platter within the drive enclosures.
While there are other form factors, by 2009, manufacturers discontinued the development of products with 1.3-inch, 1-inch and 0.85-inch form factors. The falling price of flash memory and flash drives made these other form factors almost obsolete. It's also important to note that while nominal sizes are in inches, actual dimensions are specified in millimeters.
Many solid-state drives are also designed for the HDD form factor. SSDs that fit into the same slots as HDDs generally use the SATA interface or serial attached SCSI, also known as a SAS interface, to transfer data to and from the host computing system.
Most HDDs are internal hard drives inside a computer and work as described above. However, external hard drives are also available to expand a computer's storage capacity or act as a portable device to back up data. External drives connect to a computer or device through interfaces such as USB 2.0, USB-C or with external SATA. External hard drives might also have slower data transfer rates compared to internal HDDs.
The main advantage of an external hard drive, in addition to expanding a device's storage space, includes being portable. Users can store data from multiple devices and physically bring that data with them wherever they go.
Hard disks can fail for all sorts of reasons. However, failures generally fall into the following six categories:
Some significant milestones in the development of hard disk drives include the following:
Solid-state drives are the main alternative to hard disk drives. Unlike hard disks, SSDs contain no moving parts and have lower latency than HDDs. SSDs are often favored to store critical data that must be accessed quickly and for applications with a high input/output demand.
SSDs are configured to deliver high read/write speeds for sequential and random data requests. They don't store data magnetically, so the read performance remains steady, regardless of where the data is stored on the drive. SSDs also have faster boot times. Because of these benefits and higher likelihood of breakdowns with hard disk drives, SSDs are replacing HDDs in many applications.
Non-volatile memory express, or NVMe SSD technology has also changed the storage landscape, including how HDDs are used. It provides faster data access and transfer speeds compared to traditional HDDs, leading to improved performance and user experience.
However, even though most PC users have started to favor SSDs, HDDs and magnetic tape are still used frequently to store large quantities of data. This is partly because SSDs are more expensive than HDDs from a price-per-gigabyte standpoint. Many enterprise storage arrays ship with a mix of HDDs and SSDs to reduce costs while providing better performance. SSDs also have a set life expectancy, with a finite number of write cycles before performance slows. Compared to an HDD, SSDs fail faster.
A hybrid hard drive is an approach that combines the strengths of both HDDs and SSDs, aiming to address the drawbacks of each. Specifically, a hybrid hard drive combines the speed of SSD read/write operations with HDD storage capacity. Hybrid hard drives feature an intelligent algorithm that stores data based on user habits. Data used often is stored in the SSD, while data accessed less often is stored in the HDD component.
This combination is beneficial for performance improvements. It is also cheaper to purchase a single hybrid hard drive than to separately purchase an SSD and an HDD. While a hybrid hard drive might seem like a silver bullet due to its high performance and durability, it is nonetheless prone to mechanical failures, especially during outages. These drives also usually come with limited storage space.
Multiple best practices must be followed to improve the performance of an HDD:
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24 Feb 2025