Early PCs did not come with internal storage
devices due to the expense. Instead, they generally had some
form of a floppy disk reader, such as those used to read 5 1/4″
floppy disks, initially labeled as “A” in MS-DOS and certain
other operating systems.
Some systems came with two such floppy disk drives necessitating
the need for a “B”. When the 3.5″ floppy disk was commonly
added, using both “A” and “B” for floppy drives was firmly
entrenched.
When hard disk drives became standard in most PCs during the
1980s, since the first two letters were already commonly used
for these floppy drives, they logically labeled the third
storage device “C”, even though it now tended to be the main
storage for the computer.
Even though no longer used, the drive designation remains, with
A and B not used. Now you can easily change, remove or add drive
letters for both physical and logical drives.