ubuntu@ubuntu:~$ sudo grub-install --root-directory=/mnt/tmp /dev/sda
Probing devices to guess BIOS drives. This may take a long time.
Installing GRUB to /dev/sda as (hd0)...
Installation finished. No error reported.
This is the contents of the device map /mnt/tmp/boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.

(fd0)	/dev/fd0
(hd0)	/dev/sda
(hd1)	/dev/sdb
(hd2)	/dev/sdc
ubuntu@ubuntu:~$