for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
sudo update-grub

grub> ls
grub> ls (hd0, mmcblk0p5)  // ext4 root partition
grub> set root=(hd0, mmcblk0p5)
grub> set prefix=(hd0, mmcblk0p5)/boot/grub
grub> insmod normal
grub> normal
(... boot ...)
sudo grub-install /dev/mmcblk0
sudo update-grub

sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
sudo grub-install --boot-directory=/mnt/boot --bootloader-id=ubuntu  --target=x86_64-efi --efi-directory=/mnt/boot/efi
shutdown -r now

• (Re)Install Grub2

  sudo su
  mount /dev/sdb1 /mnt
  mount --bind /dev /mnt/dev         &&  mount --bind /dev/pts /mnt/dev/pts
  mount --bind /proc /mnt/proc       &&  mount --bind /sys /mnt/sys
  chroot /mnt
  grub-install /dev/sdb
  grub-install --recheck /dev/sdb
  update-grub
  exit
  umount /mnt/sys       &&  umount /mnt/proc
  umount /mnt/dev/pts   &&  umount /mnt/dev   &&  umount /mnt

sudo rsync -aAXv /src --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found"} /dest