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