That means of course that 'fftl' command no longer works for the O750t ramloader (boot.bin/Ldr.bin) and has been replaced by the command 'ffs'. Maybe it still works for O6x0's ramloader but that's moot at present because the device seems unwilling to load the unmodified O6x0 ramloader.

Successful erasure of the file system resulting in no change is consistent with the 'Volume Not Mounted' error and i doubt there was any usable/recoverable data present previously and there certainly isn't now. Try to open in a Hex Editor the 48 or 83 BIN you previously dumped. I think there will only be gibberish or empty FFs bytes in there.

You've established previously that O6x0's ramloader doesn't seem to work now. It may be simply because the actual process of booting involves the firmware in region 14 and that may or may not be O750t's fw_all.bin as complete data. Even if it is complete it's clearly not working on that hardware. We can modify O6x0's ramloader so that it doesn't need the fw in 14 but then it can't work as Ldr.bin in a card. However it might work as an RGN in preboot to flash regions 12 and 14 with O6x0 firmware via text commands. Read [Only registered and activated users can see links. ] in "Running programs from SD on nuvi" thread. If you want to try but need help compiling the files let me know.