Well even if it does have a pre-boot mode as in nuvi-type units, without a USB or Serial port it's not usable to load anything anyway.

With a flash drive that big it's obvious that it's intended for the unit to hold more than one detail chart so there should be a way to add/overwrite maps.

The only obviously possible way to me would be by writing from an SD using text commands [update.txt] to transfer files from the card to the internal flash memory. Therefore download that massive software pack and look for your unit's firmware. Once you have that and confirm it's the usual Garmin proprietary OS you can check the fw in hex to see if it support the various commands mentioned in the first post of that thread i linked [e.g. rrgn, xrgn, copy etc]. Then it should be an easy task to extract the boot.bin from the firmware to use as ldr.bin to execute commands. With a large SDXC card [probably need to reformat from exFAT to FAT32] you could dump all the flash memory's regions 1 to 255, the largest of them [probably 48 or 83] will be the unit's internal file system so you'll have its structure and the map placement and naming conventions. The file system bin file can be opened with a program like PowerISO to view and save the actual files. Then writing a new chart back should also be possible.