BPI-R3 won't boot - Loading MMC instead of SD

If you format with NTFS that implies you are creating a single partition covering all the card. As GPT is stored in the beginning as well as the end of the disk, most likely the backup GPT at the end of the disk survived the subsequent overwriting with the OpenWrt image…

On Windows I recommend to use Balena Etcher or similar tool to be on the safe side.

The failing image was creaed using dd command on my Mac which suffered form a heart attack. On Windows I used Balena but even after burning again yesterday / forelast day it kept failing. Only with todays snapshot it worked.