- External hard disk drive
An external hard disk drive is a type of
hard disk drivewhich is externally connected to a computer. Modern entries into the market consist of standard SATA, IDE, or SCSIhard drives in portable disk enclosures with SCSI, USB, IEEE 1394 Firewire, eSATAclient interfaces to connect to the host computer.
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 Macintoshcomputers 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 ProFilewere the only reasonable option. Early external drives were not as compact or portable as their modern descendents.cite web|url = http://www.disktrend.com/5decades2.htm | title= Five decades of disk drive industry firsts | publisher = disktrend|accessdate = 2008-01-10] cite web|url = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_3340.html | title= IBM 3340 direct access storage facility | publisher = IBM|accessdate = 2008-01-10] cite web|url = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_1301.html | title= IBM 1301 disk storage unit | publisher = IBM|accessdate = 2008-01-10]
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.
tructure and design
The internal structure of external hard disk drives is similar to normal hard disk drives; in fact, they include a normal hard disk drive which is mounted on a
disk enclosure. In a 2007 PC Magazinearticle comparing the top 10 external hard drives, they found the cost per gigabyte value varied between ~0.30-0.50 USD.cite web|url = http://www.pcworld.com/article/id,123728/article.html|title = Top 10 External Hard Drives| publisher = PC Magazine| date = 16 November 2007|accessdate = 2007-12-29] As external hard drives retain the platters and moving heads of traditional hard drivesthey are much less tolerant of physical shocks than flash-based technology. [cite web|url = http://www.storagesearch.com/olixir-art1.pdf | title = What makes a great external hard drive| type = PDF | accessdate = 2007-12-29] Larger models often include full-sized 3.5" PATA or SATA desktop hard drives, are available in the same size ranges, and generally carry a similar cost. More pricey models, especially small USB-powered pocket hard drives, generally cost considerably more per gigabyte and are limited to the sizes available for smaller drives intended for laptop and embedded devices. Some achieve sizes little different from small MP3 players, built around CompactFlash microdrives. [ [http://www.ritzcamera.com/product/812042612.htm Pexagon Store-It 4GB Pocket Microdrive USB Storage Drive - Ritz Camera] ]
Modern external hard drives are compatible with all
operating systems supporting the relevant interface standards they operate with, such as USB MSC or IEEE1394. These standards are supported by all major modern server and desktop operating systems and many embedded devices. Obsolete systems such as Windows 98[http://www.querycat.com/faq/ecdfeb8a60e2616cb47e1546f7833093] and bundled backup and encryption software may require the installation of additional software. The drives are partitioned and formatted in the same manner as internal hard disk drives: Given the lack of any universally supported filesystem (barring the obsolete FATfilesystem family), owners often need to reformat these drives upon purchase to achieve full compatibility with operating systems such as Mac OS Xand Linux.
USB flash drive
* [http://www.pcmag.com/article2/0,1759,1912905,00.asp PC Magazine reviews about external hard disk drives]
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