I installed the latest RetroFW tonight from the google drive link in the OP, using a 64GB internal SD card, and I used a 256GB SD card as a ROM disk in the cartridge adapter.
The 256GB SD card does not mount in retrofw, but it worked just fine in uselessFW.
Also, plugging my RS97 with RetroFW into my computer does not mount/detect any drives in windows (though the device drivers appear to work just fine, and windows recognizes the connection). Not sure if these are known issues, just wanted to report them.
I was wrong about the 256GB SD card, it is mounted in RetroFW, but its in the /media directory not the /mnt directory.
However using a 64GB SD card as the internal seems to have lead to lots of problems, including an inability to write anything to the card from within gmenu2x, ie can't install any ipk or modify any settings. I installed RetroFW on the original 16GB SD card and it resolved my issues, but introduced a few new ones...