I have two MacBooks. One installed Ubuntu 18.04 flawlessly. The other would hang on the Splash Screen for the intial install. So in the grub menu, I pressed 'e' and typed in nomodeset where applicable. That did the trick, and I was able to install Ubuntu. Then I updated the graphics to NVIDIA, rebooted, and now I'm getting a blank screen on boot-up. If I press option + command + r, I'm able to get to the grub menu again (not from the boot USB), and if I press e, I see that nomodeset is already there. I don't understand what to do at this point.
I also tried to start completely over, and press option + power to boot to USB, and my USB drive won't show up in the menu! I tried different USB drives, and same result.