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
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.
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