In them olden days, when we first had 'puters, they were run from "floppy" disks - large floppy storage media containing the operating system, programs etc. It soon became obvious that a storage data area was required (A and B drives). Technology progressed and the whizz kids invented permanent, built in drives (C drive) possibly partitioned (split into C and D). The large floppy disks had by now been replaced by the smaller more robust, disks (which although no longer floppy were still called floppies).
Networking developed and computer whizzies continued with the Alphabet labelling of drives - in my office, G is the global drive, Y is your own drive etc. On stand alone or non-networked computers, G is usually one of the USB ports you can plug your camera cable etc into. I can't remember what happened to E and F though.....