I have a mid-2012 non retina Macbook Pro. I am trying to dual boot ubuntu, and have followed the instructions outlines on this page: https://www.lifewire.com/dual-boot-l...mac-os-4125733

However, when I try to boot into my Live USB from REFInd it immediately sends me to a black screen with a blinking underscore (so I can't install ubuntu or try it out from the USB). Does anyone know what the problem might be?