ubuntu@ubuntu:~$ sudo mount /dev/sda3 /mnt
ubuntu@ubuntu:~$ sudo mount -o bind /dev /mnt/dev
ubuntu@ubuntu:~$ sudo mount -o bind /sys /mnt/sys
ubuntu@ubuntu:~$ sudo mount -t proc /proc /mnt/proc
ubuntu@ubuntu:~$ sudo cp /proc/mounts /mnt/etc/mtab 
ubuntu@ubuntu:~$ sudo chroot /mnt /bin/bash 
root@ubuntu:/# grub-install /dev/sda
Could not find device for /boot: Not found or not a block device.
root@ubuntu:/# grub-install --recheck /dev/sdX
Probing devices to guess BIOS drives. This may take a long time.
/dev/sdX: Not found or not a block device.
root@ubuntu:/# grub-install --recheck /dev/sda
Probing devices to guess BIOS drives. This may take a long time.
Could not find device for /boot: Not found or not a block device.