A hard disk drive (HDD)
is a data storage device used for storing and
retrieving digital information using rapidly rotating
disks (platters) coated with magnetic material. An HDD
retains its data even when powered off. Data is
read in a random-access manner, meaning individual blocks of data can be stored or retrieved
in any order rather than sequentially.
An HDD consists of one or more rigid ("hard") rapidly rotating disks
(platters) with magnetic heads arranged on a moving actuator
arm to read and write data to the surfaces.
Introduced by IBM in 1956, HDDs became
the dominant secondary storage device for general-purpose computers
by the early 1960s. Continuously improved, HDDs have maintained this position
into the modern era of servers and personal
computers. More than 200 companies have produced HDD units, though
most current units are manufactured by Seagate, Toshiba
and Western Digital. Worldwide disk storage
revenues were US $32 billion in 2013, down 3% from 2012.
The primary characteristics of an
HDD are its capacity and performance. Capacity is
specified in unit prefixes corresponding to powers of 1000:
a 1-terabyte
(TB) drive has a capacity of 1,000 gigabytes
(GB; where 1 gigabyte = 1 billion bytes). Typically, some of an HDD's capacity is unavailable to
the user because it is used by the file system
and the computer operating system, and possibly inbuilt
redundancy for error correction and recovery. Performance is
specified by the time required to move the heads to a track or cylinder
(average access time) plus the time it takes for the desired sector to move under
the head (average latency, which is a function of the
physical rotational speed in revolutions per minute), and finally the
speed at which the data is transmitted (data rate).
The two most common form factors
for modern HDDs are 3.5-inch
in desktop computers and 2.5-inch in laptops. HDDs are connected to systems by
standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.
As of 2015, the primary competing
technology for secondary storage is flash memory
in the form of solid-state drives (SSDs). HDDs are the
dominant medium for secondary storage due to advantages in price per unit of
storage and recording capacity. However, SSDs are replacing HDDs where speed, power consumption and durability are more
important considerations.
HDDs were introduced in 1956 as
data storage for an IBM real-time transaction processing computer and were
developed for use with general-purpose mainframe and minicomputers.
The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and
stored 5 million 6-bit characters (3.75 megabytes)
on a stack of 50 disks.
In 1962 IBM introduced the model 1311
disk drive, which was about the size of a washing machine and stored two
million characters on a removable disk pack.
Users could buy additional packs and interchange them as needed, much like
reels of magnetic tape. Later models of removable pack
drives, from IBM and others, became the norm in most computer installations and
reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were
called "fixed disk" drives.
Some high performance HDDs were
manufactured with one head per track, e.g., IBM 2305
so that no time was lost physically moving the heads to a track. Known as
Fixed-Head or Head-Per-Track disk drives they were very expensive and are no
longer in production.
In 1973, IBM introduced a new type
of HDD codenamed "Winchester". Its primary distinguishing feature was
that the disk heads were not withdrawn completely from the stack of disk
platters when the drive was powered down. Instead, the heads were allowed to
"land" on a special area of the disk surface upon spin-down,
"taking off" again when the disk was later powered on. This greatly
reduced the cost of the head actuator mechanism, but precluded removing just
the disks from the drive as was done with the disk packs of the day. Instead,
the first models of "Winchester technology" drives featured a
removable disk module, which included both the disk pack and the head assembly,
leaving the actuator motor in the drive upon removal. Later "Winchester"
drives abandoned the removable media concept and returned to non-removable
platters.
History
Like the first removable pack
drive, the first "Winchester" drives used platters 14 inches
(360 mm) in diameter. A few years later, designers were exploring the
possibility that physically smaller platters might offer advantages. Drives
with non-removable eight-inch platters appeared, and then drives that used a 5 1⁄4 in
(130 mm) form factor
(a mounting width equivalent to that used by contemporary floppy disk
drives). The latter were primarily intended for the then-fledgling
personal computer (PC) market.
As the 1980s began, HDDs were a
rare and very expensive additional feature in PCs, but by the late 1980s their
cost had been reduced to the point where they were standard on all but the
cheapest computers.
Most HDDs in the early 1980s were
sold to PC end users as an external, add-on subsystem. The subsystem was not
sold under the drive manufacturer's name but under the subsystem manufacturer's
name such as Corvus Systems and Tallgrass Technologies, or under the PC
system manufacturer's name such as the Apple ProFile.
The IBM PC/XT
in 1983 included an internal 10 MB HDD, and soon thereafter internal HDDs
proliferated on personal computers.
External HDDs remained popular for
much longer on the Apple Macintosh. Every Mac made between 1986
and 1998 has a SCSI
port on the back, making external expansion easy; also, "toaster"
Compact Macs did not have easily accessible HDD bays (or, in the case of the Mac Plus,
any hard drive bay at all), so on those models, external SCSI disks were the
only reasonable option.
The 2011 Thailand floods damaged manufacturing
plants, and impacted hard disk drive cost adversely in 2011-2013.
Driven by ever increasing areal density since their invention, HDDs
have continuously improved their characteristics; a few highlights are listed
in the table above. At the same time, market application expanded from mainframe computers of the late 1950s to most mass storage
applications including computers and consumer applications such as storage of
entertainment content.
Keyboard QWERTY : ditemukan oleh Scholes, Glidden dan Soule
pada tahun 1878, dan fungsi keyboard QWERTY ini digunakan sebagai standar mesin
tik komersial pada tahun 1905.
Keyboard Alphabetik : keyboard alphabetik disusun persis seperti
pada tata letak QWERTY maupun Dvorak, tetapi susunan hurufnya berurutan seperti
pada urutan alphabet