The Garmin will only act like a usb card reader when it's in Mass Storage Mode (showing in Disk Management and File Explorer). It can't do that when it's connected to the PC in preboot.

So here's the SD card kit to try 'dumping' (i.e. copying) it's non-volatile memory region. Look behind the Spoiler:
Spoiler: Click
[Only registered and activated users can see links. ]
  • Extract the above ZIP file directly to a CLEAN microSD card at least of 8GB size, formatted only as FAT (FAT16 or FAT32, not exFAT)
  • Make sure the device is fully OFF (force it off by holding the power button down if necessary)
  • Insert the card then power it on
  • "Loader" should appear on the screen, wait for the flash to finish before removing the card.

If successful there will be a new file named 154.bin in the root of the card alongside the Garmin folder. Even if it doesn't work there's one more thing we can try so just report back the result and also say what's in a new file named 'update.log' if it's indeed created in the 1758 folder.