For a very long period of time,  the primary hard disk has always been designated with the alphabet letter “C”. This dates back to the days when Hard disk drives were placed in personal computers running certain popular operating systems most especially MS-DOS/windows and this is why.

In the genesis of personal computers, the PCs never came with hard drives. Before long, the most notable form of mass storage comprised of floppy disc drives that were availed for PCs. The first floppy disk drives were external drives and they functioned similarly to the way modern USB hard drives work. They were just plugged in with a cable and ready to use. Primarily, the first floppy drive connected to the computer was recurrently referred to as drive 0 and in case there was another drive it then became drive 1. Over time, the IBM introduced Microsoft’s DOS (PC-DOS and MS-DOS) which relatively changed the way home computer’s designated floppy drives. In other words, where we once had drive 0 and drive 1, it turned into drive A and drive B respectively. It’s been known to be that way since back in the day.

Eventuality, drives became available for home computers and they were referred to as drive C since A and B were already in use by floppy disc drives. As time went on, CD and DVD came on board and were known as drive D. With the invention of the optical drives, the floppy drives went down the drain as they were no longer being produced and surprisingly by that era everyone was used to the hard drive being drive C and optical drive being drive D. Progressively, the floppy drives like I said earlier went down the drain, alongside the drive letter A and B that referred to them.

In the present day, PC has connectors for a variety including SD cards, flash discs, external USB drives, etc. and because of this it can use any number of drive letters and of course each is given a unique drive letter.

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source: fossbytes