A simpler change would be to to add "|| true" to the end of the "GRUB_DEVICE=..." line in grub-mkconfig, and then set your root device by hand in /etc/default/grub, or something along those lines.
A simpler change would be to to add "|| true" to the end of the "GRUB_DEVICE=..." line in grub-mkconfig, and then set your root device by hand in /etc/default/grub, or something along those lines.