#2 Computer Storage

 Hard Disk Storage

Spinning hard disk (HD) drives are today the most common means of high capacity computer storage, with most desktop and laptop computers still relying on a spinning hard disk to store their operating system, applications programs and at least some user data. Traditional, spinning hard disk drives consist of one or more disk "platters" stacked one above the other, and coated in a magnetic media that is written to and read by the drive heads. As discussed in the hardware section, hard disk drives can transfer data directly to other computer hardware via a range of three interface types (SATA, IDE/UDMA, or SCSI) and come in a range of speeds from 4200 to 15000 revolutions per minute (RPM).

Hard disks are almost always manufactured with either 3.5" of 2.5" platters (although just to break the rule a few smaller -- most notably 1.8" -- and even some larger platter disks are made by some manufacturers). For many years 3.5" hard disks have been standard for desktop computers and servers, and 2.5" hard disks for laptops. Yet this is now starting to change, with enterprise class 2.5" hard disks now increasingly being used in servers and some desktop computers due to their low power requirements. Indeed, the fact that Western Digital's top-of-the-range Velociraptor hard drives now use a 2.5" rather than a 3.5" mechanism speaks volumes and probably indicates that within a few years most spinning hard disk drives are likely to be 2.5". (Note that some raptor models are supplied in a metal "sled" for fitting into a 3.5" bay)

Whilst at least one hard disk is usually required inside a computer as the "system disk", additional hard disk drives can be located either "internally" inside the main computer case, or connected "externally" as an independent hardware unit. A second internal hard disk is highly recommended where a user regularly works on very large media files (typically digital video files) that are always accessed directly off the hard disk, rather than loaded into RAM. Where such files are loaded off a computer's system disk, the disk drive heads are inevitably constantly nipping back and forth between accessing the large media file and writing temporary operating system files, and this both degrades performance and reduces the life of the disk.

source explainingcomputers