I messed up. I installed mint18 on my external drive but mint18 is not quite ready for prime time so I figured I would put mint 17.3 on. Well, durring installation I told it to put grub on the external drive. now I can't boot unless the external drive is plugged in. I need the port the external is plugged into because that is the only USB port it will boot from and I need that port  for the thumb drive. So  my question is how do I install grub to sda1......