Installing GRUB after the fact

After successfully getting Sierra up and running on my old Gigabyte box, I'm now attempting to dual-boot Ubuntu 16.04, using the second 80 GB HD I had used to beta test Sierra. In a sense, that's going back to this machine's roots, as Ubuntu was the OS originally installed when I bought it.

I used the Ubuntu "mini" net installer on a CD, as I'd used that to install Xubuntu on an...

Installing GRUB after the fact