alt Hacker NewsEven further back into time, before 3.5" disks, both A: and B: were 5.25" disks. And, although my memory is hazy, 3.5's were commonly slotted into B: at first, because no one had 3.5" boot disks until the drives became somewhat common.
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raldi • today at 5:07 PM
The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them, though everybody had at least one floppy.
Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)
You are forgetting 8” floppies. IBM used only the soft sector ones (one hole punched close to the spindle to mark sector 0) but there were also hard sector ones (a ring of 32 holes close to the spindle hole).
There was a lot of experimentation back in those days. The Wikipedia’s page on floppy disks is surprisingly long!