A hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive,is a Non volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media. 
Originally, the term "hard" was temporary slang, substituting "hard" for "rigid", before these drives had an established and universally-agreed-upon name. A HDD is a rigid-disk drive although it is rarely referred to as such. By way of comparison, a floppy drive (more formally, a diskette drive) has a disc that is flexible. Some time ago, IBM's internal company term for a HDD was "file".
HDDs (introduced in 1956 as data storage for an IBM accouting computer) were originally developed for use with general purpose computers, see History of hard disk drives.
In the 21st century, applications for HDDs have expanded to include digital video recorders , digital audion players, personal digital and video game consoles . In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attched storage(NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data. Note that although not immediately recognizable as a computer, all the aforementioned applications are actually embedded computing devices of some sort.
Originally, the term "hard" was temporary slang, substituting "hard" for "rigid", before these drives had an established and universally-agreed-upon name. A HDD is a rigid-disk drive although it is rarely referred to as such. By way of comparison, a floppy drive (more formally, a diskette drive) has a disc that is flexible. Some time ago, IBM's internal company term for a HDD was "file".
HDDs (introduced in 1956 as data storage for an IBM accouting computer) were originally developed for use with general purpose computers, see History of hard disk drives.
In the 21st century, applications for HDDs have expanded to include digital video recorders , digital audion players, personal digital and video game consoles . In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attched storage(NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data. Note that although not immediately recognizable as a computer, all the aforementioned applications are actually embedded computing devices of some sort.
The earliest “form factor” hard disk drives inherited their dimensions from flopy-diskso that either could be mounted in chassis slots, and thus the HDD form factors became colloquially named after the corresponding FDD types. "Form factor" compatibility continued after the 3½ inch size even though floppy disk drives with new smaller dimensions ceased to be offered.
8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)In 1979, Shugart SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD. Both "full height" and "half height" (2.313 in) versions were available.
5.25 inch: 5.75 in × 1.63 in × 8 in (146.1 mm × 41.4 mm × 203 mm)This smaller form factor, first used in an HDD by Seagate in 1980, was the same size as full height 5¼-inch diameter FDD, i.e., 3.25 inches high. This is twice as high as "half height" commonly used today; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks DVD ,CD use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quatum Bigfoot HDD was the last to use it in the late 1990s, with “low-profile” (≈25 mm) and “ultra-low-profile” (≈20 mm) high versions.
3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm)This smaller form factor, first used in an HDD by Rodime in 1984, was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today has been largely superseded by 1-inch high “slimline” or “low-profile” versions of this form factor which is used by most desktop HDDs.
2.5 inch: 2.75 in × 0.374–0.59 in × 3.945 in (69.85 mm × 9.5–15 mm × 100 mm)This smaller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives. Today, the dominant height of this form factor is 9.5 mm for laptop drives, but high capacity drives have a height of 12.5 mm. Enterprise-class drives can have a height up to 15 mm.
1.8 inch: 54 mm × 8 mm × 71 mmThis form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It is increasingly used in digital audio players and subnotebooks. An original variant exists for 2–5 GB sized HDDs that fit directly into a PC Card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
1 inch: 42.8 mm × 5 mm × 36.4 mmThis form factor was introduced in 1999 as IBM's Microdive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.0.85 inch: 24 mm × 5 mm × 32 mm Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version and holds the Guiness for the smallest harddisk drive.
Major manufacturers discontinued the development of new products for the 1-inch (1.3-inch) and 0.85-inch form factors in 2007, due to falling prices of Flash memor,although Samsung introduced in 2008 with the SpinPoint A1 another 1.3-inch drive.
The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.
8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)In 1979, Shugart SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD. Both "full height" and "half height" (2.313 in) versions were available.
5.25 inch: 5.75 in × 1.63 in × 8 in (146.1 mm × 41.4 mm × 203 mm)This smaller form factor, first used in an HDD by Seagate in 1980, was the same size as full height 5¼-inch diameter FDD, i.e., 3.25 inches high. This is twice as high as "half height" commonly used today; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks DVD ,CD use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quatum Bigfoot HDD was the last to use it in the late 1990s, with “low-profile” (≈25 mm) and “ultra-low-profile” (≈20 mm) high versions.
3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm)This smaller form factor, first used in an HDD by Rodime in 1984, was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today has been largely superseded by 1-inch high “slimline” or “low-profile” versions of this form factor which is used by most desktop HDDs.
2.5 inch: 2.75 in × 0.374–0.59 in × 3.945 in (69.85 mm × 9.5–15 mm × 100 mm)This smaller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives. Today, the dominant height of this form factor is 9.5 mm for laptop drives, but high capacity drives have a height of 12.5 mm. Enterprise-class drives can have a height up to 15 mm.
1.8 inch: 54 mm × 8 mm × 71 mmThis form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It is increasingly used in digital audio players and subnotebooks. An original variant exists for 2–5 GB sized HDDs that fit directly into a PC Card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
1 inch: 42.8 mm × 5 mm × 36.4 mmThis form factor was introduced in 1999 as IBM's Microdive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.0.85 inch: 24 mm × 5 mm × 32 mm Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version and holds the Guiness for the smallest harddisk drive.
Major manufacturers discontinued the development of new products for the 1-inch (1.3-inch) and 0.85-inch form factors in 2007, due to falling prices of Flash memor,although Samsung introduced in 2008 with the SpinPoint A1 another 1.3-inch drive.
The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.
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