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The Hard Disk Drive.

ibm romac

HDDs were introduced in 1956 as data storage for an IBM real-time computer. 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 disks.

Hard Disk Early Application.

In 1961 IBM introduced the 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 tapes. 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.

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Hard Disk Drive Modern Design.

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 Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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 5 1⁄4 in (130 mm) form factor (a mounting width equivalent to that used by a floppy disk drive. The latter were primarily intended for the then-fledgling personal computer (PC) market.

Hard Drive Market Development.

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 System and Tallgrass Technology, or under the PC system manufacturer's name such as the Apple Pro file. 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 Mackintosh. 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 Hard Drive as we know it today.

Driven by areal density doubling every two to four years since their invention (an observation known as Kryders law, similar to Moores Law , HDDs have continuously improved their characteristics; a few highlights include:

• Capacity per HDD increasing from 3.75 megabytes to 4 terabytes or more, more than a million times larger.

• Physical volume of HDD decreasing from 68 cubic feet (1.9 m3) (comparable to a large side-by-side refrigerator), to less than 20 milli-litres (0.70 imp fl oz; 0.68 US fl oz) a 100,000-to-1 decrease.

• Weight decreasing from 2,000 pounds (910 kg) to 48 grams (1.7 oz), a 20,000-to-1 decrease.

• Price decreasing from about £10,000 per megabyte to less than £0.00006 per megabyte (£50/1.5 terabyte), a greater than 250-million-to-1 decrease.

• 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.

The picture below shows a modern slim design capable of storing over one million times more data than the 350 ROMAC shown at the top of the page and retrieving that data a million times faster.

Hard Disk Drive Technology.

A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is 0.07–0.18 mm (70,000–180,000 nm).

A hard disk has one or more platters - or disks - and each platter usually has a head on each of its sides. In modern drives the platters are made from glass or ceramic. The platters themselves are non-magnetic but have a magnetic coating which holds the magnetic impulses which represent the data. A typical hard drive will have three or four platters and modern varieties can hold 20GB per disk.

Platter Rotational Speeds.

The platters in contemporary HDDs are spun at speeds varying from 4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. Today, the platters in most consumer HDDs spin in the range of 5,400 rpm to 7,200 rpm.

Hard Disk Read/Write Heads.

The read/write heads float on a cushion of air only nanometers above the surface of the platters. As the read/write heads pass over the spinning platters they magnetize the surface in a pattern which represents the data in digital form. Information is written to and read from a platter as it rotates past devices called read/write heads that operate very close (often tens of nanometers) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. In modern drives there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track but modern drives (since the 1990s) use zone bit recording—increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones.

Hard Disk Data Recording + Density.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom layer of the non-magnetic element ruthenium , and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is perpendicular , first shipped in 2005, and as of 2007 the technology was used in many HDDs.

The data is stored digitally as tiny magnetized regions, called bits, on the disk. A magnetic orientation in one direction on the disk could represent a "1", while an orientation in the opposite direction could represent a "0".

Data is arranged in sectors along a number of concentric tracks. These tracks are arranged from the inner diameter of the disk to its outer edge.

When reading data on a disk, a similar process occurs in reverse.

Hard Disk Drive Spindle.

The platters are mounted on the spindle which is turned by the drive motor. Most current hard disk drives spin at between 5,400 and 10,000 RPM. Modern hard drives can transfer 80 megabytes of data per second.

Hard Disk Head Arm.

The triangular-shaped head arm holds the read/write heads and is able to move the heads from the hub to the edge of the drive.

There is one hard arm per read/write head and all of them are lined up and mounted to the actuator as a single unit.

Voice Coil Actuator.

The head arm is controlled by an actuator - which has to be incredibly accurate. 30,000 tracks can be stored within one inch of space on a platter.
The hard disk's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller via its interface to the computer.

Air Filter.

Hard drives are air-sealed to enable the heads to float and to avoid contamination from dust.

The air inside the the hard drive enclosure is in constant motion and passes through the filter to remove any leftover contaminants from the manufacturing process and any particles or chemicals that may have somehow entered the enclosure.
A HDD records data by magnetizing a thin film of magnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, which determines how the data is represented by the magnetic transitions.

*see also helium filled hard drives:-
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