RAW format means a disc isn't formatted properly or maybe it's displaying a card reader that's empty. Are you sure that's the nuvi showing with Drive letter F? You said in RMPrepUSB it's displaying as 'unmounted' which means it hasn't got a drive letter allocated. Did you allocate 'F' in disc management to it, because if you didn't that's not it.
Look in Device Manager to see if it's displayed as a Garmin device there. Start again: Cure it. Do a format with RMPrepUSB to FAT32. Load original fw and then report back.