Going to show my age again now. I think post 1990, they were rare. Essentially there was a time before that though, when computers often didn't have a hard drive. Because they were like 10MB and cost thousands of whatever your currency may be. So in those days (8088/8086 based systems largely), it was common for there to be 2x5.25" drives. A: and B:. C: was a status symbol, the lambo of the early computer era. As hard drives became more affordable (for levels of affordability), there was also the rise of the new disk type. 3.5". In that case it wasn't too uncommon to have a 3.5" drive, a 5.25" drive and a hard disk. So you'd have A:, B: and C:. I think after the mid 90s the much more common setup was a single high density 3.5" drive A: and a hard disk C:. In the office we also had an F: drive (Novell netware network drive). A: and B: were by and large kept free out of that history. I am pretty sure you can map HDDs/SSDs to them these days though if you want to.