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[[File:Hard disk drive inside.jpg|thumbnail|A view inside a 3.5 inch hard disk drive (this is usually covered by metal panel)]]

[[File:Hard disk drive inside.jpg|thumbnail|A view inside a 3.5 inch hard disk drive (this is usually covered by metal panel)]]

A '''hard drive''' is an older digital storage device for computers.  It is typically a set of mechanical rotating disks ("platters") with read/write "heads" that "fly" at sub-micron heights above them, reading and writing data magnetically.  The heads typically "seek", moving over the disk's surface to access the desired circular track on the disk.  This is in contrast to the [[RAM]], random access memory that the computer accesses on a nanosecond-by-nanosecond basis.  The disk drive is typically more than a million times slower than the RAM, but with much larger data capacity.  It also has the property that its data persists when the computer is turned off, while RAM often loses its data.  When the computer is operating, it requires access to both the RAM and the disk.  The [[operating system]] performs operations to read data from the disk into the RAM, and proceeds from there.

A '''hard drive''' is an older digital storage device for computers.  It is typically a set of mechanical rotating disks ("platters") with read/write "heads" that "fly" at sub-micron heights above them, reading and writing data magnetically.  The heads typically "seek", moving over the disk's surface to access the desired circular track on the disk.  This is in contrast to solid-state memory such as [[RAM]] (random access memory) that the computer accesses on a nanosecond-by-nanosecond basis.  The disk drive is typically more than a million times slower than the RAM, but with much larger data capacity.  It also has the property that its data persists when the computer is turned off, while RAM loses its data without sustained power.  When the computer is operating, it requires access to both the RAM and the disk.  The [[operating system]] performs operations to read data from the disk into the RAM, and proceeds from there.

The computer's programs and files are permanently stored on the disk.

The computer's programs and files are permanently stored on the disk.

When a computer is "suspended", the operating system stops running and the processor is powered off, but active data is still left in the RAM, so the computer can be restarted nearly instantly.  A small amount of power from the battery is required to keep the RAM data from evaporating.  When a computer is "hibernated", the active data from the RAM is written so a special place on the disk, and the computer is powered off completely.  Saving and restoring the RAM takes several seconds.

When a computer is "suspended", the operating system stops running and the processor is powered off, but active data is still left in the RAM, so the computer can be restarted nearly instantly.  A small amount of power is required to keep the RAM data from evaporating.  When a computer is "hibernated", the active data from the RAM is written to a special place on the disk, and the computer is powered off completely.  Saving and restoring the RAM takes several seconds.

The word "hard" refers to a time when such high-performance and high-capacity disks were a novelty, and computers only had [[floppy disk]]s as their backing storage.  These floppy disks typically had a capacity of only a megabyte or so, and the computers were incredibly primitive by today's standards.  But the word "hard", and the abbreviation "HDD" lives on.

The word "hard" refers to a time when such high-performance and high-capacity disks were a novelty, and computers only had [[floppy disk]]s as their backing storage.  These floppy disks typically had a capacity of only a megabyte or so, and the computers were incredibly primitive by today's standards.  But the word "hard", and the abbreviation "HDD" lives on.

Even when a computer is configured to start up by network boot (the [[operating system]] is retrieved from a server) a hard drive is generally needed.

Even when a computer is configured to start up by network boot (the [[operating system]] is retrieved from a server) a hard drive is generally needed.

Disk drives are often replaced with "flash ram" as the technology of the latter improves.  Such devices are often called "SSD" for solid-state drive.  Flash ram can retain its data when the power is off.  Computers using SSD can be "booted up" instantly.  The technology of flash ram has not reached the point where it can compete with disks for data capacity, so SSD computers are typically small laptops, whereas computer used for serious things like software development still use disk drives.

Disk drives are often replaced with flash memory as the technology of the latter improves.  Such a device is often called "SSD" for solid-state drive.  Flash memory can retain its data when the power is off.  However, due to the operating speed of this solid-state memory, computers using a SSD can be "booted up" almost instantly.

==Varieties==

==Varieties==

===Communication Types===

===Communication Types===

These drives can communicate with the system's [[motherboard]] using two different methods.  The older and largely obsolete type was [[Parallel ATA]] (PATA), which was formerly called IDE ([[Integrated Drive Electronics]]).  The newer and now standard type is [[Serial ATA]] (SATA).  This newer type can communicate at a greater rate to make better use of this, the rotational speed of SATA drives was also increased.  This means that data can be written and retrieved more quickly, but the drive could potentially fail sooner than it's slower predecessor.

These drives can communicate with the system's [[motherboard]] using a few different methods.  The older and largely obsolete type was [[Parallel ATA]] (PATA), which was formerly called IDE ([[Integrated Drive Electronics]]).  The newer and now standard type is [[Serial ATA]] (SATA).  This newer type can communicate at a greater rate, and to make better use of this, the rotational speed of SATA drives was also increased.  This means that data can be written and retrieved more quickly, but the drive could potentially fail sooner than it's slower predecessor.

A higher-performance interface is SCSI (Small Computer Systems Interface), configured as SAS (Serial Attached SCSI).  Such disks typically have higher performance (higher rotational speeds) than are found on ordinary laptops.

A higher-performance interface is SCSI (Small Computer Systems Interface), configured as SAS (Serial Attached SCSI).  Such disks typically have higher performance (higher rotational speeds) than are found on ordinary laptops.