The first thing one needs to know is the floppy disks were invented before the hard disks came into existence. Earlier floppy disks were used to store even the Operating System.
Yes, this is the reason. The operating system allocates the drive letter A for ” First floppy drive “ and letter B for “Second floppy drive”.It doesn’t matter whether your system has floppy drive or not, but by default this two letters are allocated for floppy drives.
A: drive is used for floppy disk of size 3.5 inches, storage capacity is 1.44MB.
B: drive is used for floppy disk of size 5.25 inches, storage capacity is 1.2.
Whenever a computer is switched on, have you noticed, the floppy drives are first checked,and then followed by the hard disk. The indicator of the floppy drives glow first. This is because the booting program checks the floppy disks first to check for any availability of programs there.
Hence it was an initial practice to start naming the floppy disk drives with the letters A and B and letter C, D, E and so on for the hard disk partitions.
It is a practice followed and it has been adopted universally for identification.
The letters A and B represent floppy drives, the letter C is reserved for the hard drive. From letter D onwards can be used for other device drives like CD ROM, USB and for hard disk partitions as well. We can uses last drive letter upto z: for mapping a drive letter for another programming purposes.