I’ve just upgraded my main system from 16.04 to 18.04. It was a fight. I have two drives and an Intel BIOS. When installing via “The something else method” the bootloader broke.
I was technically out of my depth, all I could get was a GRUB prompt.
Ubuntu-Mate solved the problem for me. Here is how that happened.
I had installed from a USB thumb drive, but I could not re-start a fresh installation attempt from it, I kept being left in front of a GRUB prompt. I had a magazine (Linux Welt) DVD with a copy of 18.04 and booted from that. This time a new installation process was launched. On the Ubuntu-Mate “Welcome” window that appeared there was a “button” to repair boot problems. I clicked it and let it run its course. I then continued with the installation process which went smoothly (except for some difficulties with my QWERTZ keyboard (for details and solution see this post):
I do not know who put that “repair boot problems” button on the Welcome screen. That was a wonderful idea. You have my gratitude.