Update-grub could not recognized as bootable a btrfs partition.
A workaround to make a btrfs (bootable) partition recognized by update-grub
is to create symlinks to /boot and /etc folders, in this simple way:
- go to the mounted btrfs partition
- create symlink for boot
ln -s @/boot boot
- create symlink for etc
ln -s @/etc etc
- do
update-grub