I finally managed to get Ubuntu installed on this mid-2010 iMac. Weird UEFI and working with grub is no joke. Now that I wrote down what I did, I want to try it all over but with Elementary OS. That was my original intent, but I switched to Ubuntu after many fails.
I know Elementary is a derivation of Ubuntu and I think my issues were not directly related to which OS. So it should work I think. Still it’s weird to break what I finally got working!
@tendigits Elementary does look good for a Mac user. It's the perfect Linux distro to make the change!
@quenquen yes! feels like a pretty smooth on-ramp.