HARD DISKS 

BY SAIF UR REHMAN KHALID

HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing computer^[1] 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 (the equivalent of 3.75 million 8-bit bytes) on a stack of 50 discs.

In 1961 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.

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 Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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.

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 5¹⁄₄ in (130 mm) form factor (a mounting width equivalent to that used by contemporaryfloppy 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 on PCs; however by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest PC.

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 10MB 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.

Driven by areal density doubling every two to four years since their invention (an observation known as Kryder's law, not unlike Moore's Law), HDDs have continuously improved their characteristics; a few highlights include:

• Capacity per HDD increasing from 3.75 megabytes^[1] to 4 terabytes or more, more than a million times larger.
• Physical volume of HDD decreasing from 68 cubic feet (1.9 m³)^[1] (comparable to a large side-by-side refrigerator), to less than 20 millilitres (0.70 imp fl oz; 0.68 US fl oz),^[6] a 100,000-to-1 decrease.
• Weight decreasing from 2,000 pounds (910 kg)^[1] to 48 grams (1.7 oz),^[6] a 20,000-to-1 decrease.
• Price decreasing from about US$15,000 per megabyte^[7] to less than $0.0001 per megabyte ($100/1 terabyte), a greater than 150-million-to-1 decrease.^[8]
• Average access time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1 improvement.
• Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.