 Interior of a hard disk drive | |
| Date invented | December 14, 1954[1] |
|---|---|
| Invented by | An IBM team led by Rey Johnson |
| Connects to | Host adapter of system, in PCs typically integrated into motherboard. via one of: |
| Market Segments | Desktop computers Mobile computing Enterprise computing Consumer electronic |
A hard disk drive[2] (hard disk,[3] hard drive,[4] HDD) is a non-volatile storage device for digital data. It features one or more rotating rigid platters on a motor-driven spindle within a metal case. Data is encoded magnetically by read/write heads that float on a cushion of air above the platters, with modern storage capacity measured in gigabytes and terabytes.
The first hard disks were invented by IBM in 1956. They have fallen in size and cost over the years, displacing floppy disks in the late 1980s as the preferred long-term storage mechanism for personal computers. Most desktop systems today have standardized on the 3.5" form factor, and though mobile systems most often use 2.5" drives, both sizes operate on similar high-speed serial interfaces.
Despite their utility, hard disk design introduces inherent performance compromises. The manipulation of sequential data depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the sole method to improve sequential transfer rates. While these advances exponentially increase storage capacity, the performance gains they enable are only linear. Performance relative to capacity in new generations of hard disks has therefore fallen with time. Operations on non-sequential data are further compromised by the overhead of moving the read/write heads to new positions. The more scattered the data, the more transfer rates suffer. The multi-layer caching structure of modern computers is a direct response to these limitations.
As of 2010, the rise of inexpensive non-volatile flash memory has made the continued dominance of hard disks in personal computers less certain. Storage capacity has increased at a rate far in excess of the data requirements of common productivity applications. Streaming media from distant servers continues to rise in popularity,[5] in league with the arrival of portable devices that require a level of power consumption, durability, and instant-access that a hard disk cannot provide. While the aggregate data demands of all users will continue to rise, future hard disk market share may shift to the enterprise sector.
External hard disk drive
An external hard disk drive is a type of hard disk drive which is connected to a computer by a USB cable or other means. Modern entries into the market consist of standard SATA, IDE, or SCSI hard drives in portable disk enclosures with SCSI, USB, IEEE 1394 Firewire, eSATA client interfaces to connect to the host computer.
History
The first commercial hard disks were large and cumbersome, were not stored within the computer itself, and therefore fit within the definition of an external hard disk. The hard disk platters were stored within protective covers or memory units, which sit outside. These hard disks soon evolved to be compact enough that the disks were able to be mounted into bays inside a computer. Early Apple Macintosh computers did not have easily accessible hard drive bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks such as the Apple ProFile were the only reasonable option. Early external drives were not as compact or portable as their modern descendents.[1] [2] [3]
By the end of the 20th century, internal drives became the system of choice for computers running Windows, while external hard drives remained popular for much longer on the Apple Macintosh and other professional workstations which offered external SCSI ports. Apple made such interfaces available by default from 1986 and 1998. The addition of USB and Firewire interfaces to standard personal computers led such drives to become commonplace in the PC market as well. These new interfaces supplanted the more complex and expensive SCSI interfaces, leading to standardization and cost reductions for external hard drives.
Hard Disk 20
 | |
| Type | Hard Disk |
|---|---|
| Release date | September 17, 1985 |
| Introductory price | 1499 [1] |
| Discontinued | September, 1987 |
| Operating system | 2.1–4.3 |
| Dimensions | plastic (Pantone 453[2] |
Features
The Hard Disk 20 (or HD20, as it was known colloquially) contained a 20MB 3.5" Rodime hard disk which provided over 50 times the data storage of the stock 400K disk drive. At a time when the average file size was around 10-20K and due to the vast number of those files the HD 20 could contain, Apple's original Macintosh File System, which did not allow for directories, would have made organizing those files unwieldy. Therefore Apple introduced it with a new System and Finder update which included the brand new Hierarchical File System allowing the user to better organize files on such a large volume. As a result only the Macintosh 512K could access it, the original Macintosh 128K did not have enough RAM to load the new file system. In fact, even for the Macintosh 512K to use the drive, it required an additional file in the System Folder on a special startup disk which added additional code into memory during startup. An ingenious startup routine also allowed the Mac to check for the presence of a System file on the Hard Disk, switch over to it and eject the startup disk. Unfortunately, the HD 20 could not be used as a startup disk directly without first loading the code from the floppy disk drive. With the release of the Macintosh Plus and the Macintosh 512Ke, both containing the upgraded 128K ROM which contained the additional code, the HD 20 could finally be used alone as a startup disk.
While other hard drives were available on the market, Apple's HD 20 was generally preferred mainly because Apple broke their own development rules when they offered it. Originally the Macintosh was designed with two serial ports which were to provide all the expansion needs required of the user. It also included a dedicated floppy disk port for one external floppy disk. Most of the hard drives which were available on the market used the slower serial port to transfer data per Apple's specifications. Apple instead engineered the HD 20 to use the faster floppy disk port, enabling the user to daisy-chain an external floppy disk drive as well as an additional HD 20. With few exceptions, this along with complete compatibility with the new Hierarchical File System, gave Apple an instant edge over the competition. In addition, the HD 20 had a convenient "zero-footprint" design which fit precisely underneath the Macintosh, merely elevating it 3 inches, but otherwise taking up no more desk-space.
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