This is the correct way to install grub2 from a Linux LiveCD/USB:
sdXY is your / Linux partition (e.g. sda1)
sdX is the hard-disk containing the linux partition (e.g. sda)
mount /dev/sdXY /mnt
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
chroot /mnt
grub-install /dev/sdX
update-grub
exit # (Ctrl+D)
umount /mnt/dev
umount /mnt/proc
umount /mnt/sys
umount /mnt
reboot
來源
