Fix "failed to connect to lvmetad"
https://askubuntu.com/questions/745218/ubuntu-wont-boot-because-of-lvmetad
vi /etc/lvm/lvm.conf use_lvmetad = 0
Links
https://wiki.gentoo.org/wiki/GRUB2/Chainloading
https://wiki.ubuntuusers.de/GRUB/GRUB-Shell/
https://wiki.archlinux.org/index.php/GRUB/Tips_and_tricks#Alternative_installation_methods