The answer goes back to the glory days of floppy discs and DOS. The early DOS operating system designated two drives, A and B, strictly for floppy drives. Why? Because many early computers didn't have native hard drives -- they booted from Drive A, and ran applications from Drive B.Later, as computers came with hard drives, the second floppy drive became a useless appendage -- the computer equivalent of an appendix. To avoid confusion during the evolutionary window when computers with new hard drives coexisted beside computers with two floppies, the hard drives were given the "C" slot.
Technically speaking, the "computer" isn't missing the B drive, it's just that later Microsoft operating systems have omitted it as unnecessary. You can read more about the ins and outs of archaic drive systems at Microsoft Support.
source: http://ask.yahoo.com/20050304.html
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