I'm sure someone will correct me if I'm wrong but, in the days when HDs were rare/very expensive, PCs had 2 floppy drives (my 1st PC was like that- with no HD). These would have been A and B drives. As hard drives became more affordable, the B drive disappeared as is now happenning with the A drive. As PCs are configured to recognise the 1st HD as C it would be a major problem to change this, so A & B are a dying breed!!