ubuntu@ubuntu:~$ sudo chroot /mnt
root@ubuntu:/# mount -a
root@ubuntu:/# grub-install && update-grub
BootCurrent: 000B
Timeout: 1 seconds
BootOrder: 0000,0008,000A,000B,0002,0001
Boot0001* CD/DVD Drive
Boot0002* Hard Drive
Boot0008* ubuntu
Boot000A* ubuntu
Boot000B* UEFI:  USB DISK 2.0
Boot0000* steamos
Installation finished. No error reported.
Generating grub.cfg ...
Found background: /usr/share/plymouth/themes/steamos/steamos_branded.png
Found background image: /usr/share/plymouth/themes/steamos/steamos_branded.png
Found linux image: /boot/vmlinuz-3.10-5-amd64
Found initrd image: /boot/initrd.img-3.10-5-amd64
grub-probe: error: cannot find a GRUB drive for /dev/sdc1.  Check your device.map.
Found Ubuntu 16.04.2 LTS (16.04) on /dev/sdb2
done
root@ubuntu:/#