What a lot of nonsense stevie21. As Skids says, B was reserved for a second drive - regardless of type. When no second drive was fitted, you could still use it as it behaved as a virtual drive. For example, in DOS you could type "Copy A: B:" and it would copy the contents of the disk in A: to a buffer, then ask you to insert a disk in drive B:, at which point you would replace the disk with a blank to be used as the destination disk.
You're right Kerplunk, but if you wanted to copy a file from one disk to another, you could type "Copy A:\Fred.txt B:\". And be prompted to replace the disk after the file had been read.