I am copying a root FS on a different partition, hence the UUID needed to boot
from that new partition is different. I am running grub-mkconfig on the main
partition (with os-prober installed and the new partition mounted).
The resulting /boot/grub/grub.cfg is perfect; the new partition menuentry is
created as expected in the /etc/grub.d/30_os-prober section. Except for one
glitch: every UUID referenced in that menuentry were updated EXCEPT the most
important one; the linux kernel command line argument root= still has the
original UUID. Manually updating it prove that the menuentry is otherwise Ok
(by booting using it).