#first I mount my ext2 partition which is on sdb partition 4
sudo mount  -o rw /dev/sdb4 /mnt
#then I install grub in my partition
sudo grub-install --root-directory=/mnt /dev/sdb
sudo chmod 777 -R /mnt
#then I configure grub to have the OS list
sudo grub-mkconfig -o /mnt/boot/grub/grub.cfg
grub
# this part is supposed to make my new grub work.
root (hd1,3) # 3 is 4 - 1
setup (hd1) # 1 is 2 for sdb - 1
quit
sudo update-grub
reboot