Did this ever get resolved? I didn't see a response to the final log.
I'm experiencing the same thing on the same platform (ASUS C302 CAVE).
Card brand doesn't seem to matter. I've tried 32GB Samsung EVO cards,
various SanDisk cards from 32GB to 128GB. None show up as boot options
in SeaBIOS. If I use the exact same data/layout except on a smaller
card like 1GB of any brand then it works no problem.
Same large cards boot fine off an external USB card reader. It appears
to be an issue with the integrated SD card reader on the C302.
I'm using the same latest MrChromeBox build from May 2017.