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.
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.
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 in a disk enclosure. In a 2009 Computer Shopper comparison of 5 top external hard drives, the capacities ranged from 160 GB to 4TB and the cost per gigabyte value varied between ~0.16-0.38 USD.[4] As external hard drives retain the platters and moving heads of traditional hard drives they are much less tolerant of physical shocks than flash-based technology (a fact often overlooked by consumers lulled into a false sense of ruggedness by rubberised styling). 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 drives with biometric security or multiple interfaces, generally cost considerably more per gigabyte. Smaller, portable 2.5" drives intended for laptop and embedded devices are slightly more expensive in cost per GB compared with larger capacity 3.5" drives. Small MP3 players, previously built around mechanical hard drive technology are now primarily solid state CompactFlash based devices.
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