update grub efi from grub pc for gpt uefi

1. make a fat32 partition of size 10M, e.g. /dev/sda1
2. sudo mkdir /boot/efi
3. put this line into /etc/fstab
   UUID=4CA9-C727  /boot/efi       vfat    umask=0077      0       1
4. sudo mount -a
5. purge old grub-pc, e.g.
6. install grub-efi. e.g.  apt install grub-efi
7. sudo update-grub
8. sudo grub-install
9. Done.

Using ubuntu live CD and suppose /mnt/2 contains all the old operation system, say ubuntu 18.04.
cd /mnt/2
mount -B /dev dev
mount -B /dev/pts dev/pts
mount -B /proc proc
mount -B /sys sys
mount -B /run run

cd /

chroot /mnt/2

vim /etc/hosts, add this line:

140.110.240.80 tw.archive.ubuntu.com 

 

To find where efi, say sda2, then

mount /dev/sda2 /boot/efi

apt purge grub-pc

apt update

apt install grub-efi

update-grub
grub-install
exit

umount /mnt/2/dev/pts
umount /mnt/2/dev
umount /mnt/2/proc
umount /mnt/2/sys
umount /mnt/2/run