I was recently showing someone how to check the space on her hard drive and she asked why is there an A drive and a C drive, but no B drive?
The reason for that, according to Ask Yahoo!, (and my own knowledge) is that on early computers (Yahoo! says MS-DOS operating system, although that’s not the only one), there were two floppy drives, therefore A and B. Hard drives were a luxury (although some computers ran on magnetic tape — yes, by that I mean cassettes!).
So you would boot up the computer from the A drive (like you can do nowadays if your system is corrupted), and use the other floppy drive, the B drive, for software. Then, when hard drives became more widely used, they created the C drive to prevent confusion.
And finally, they dropped the second floppy drive, the B drive. Some of today’s computers don’t even have a floppy drive! Mine doesn’t, although the A drive is still there.
So, there’s your answer. If you’re interested in this stuff, check out this Microsoft Support article.