I am trying to install Ubuntu 15.04 on my iMac 27". I use the full install .iso with error checking returning positive with no errors on a PC, and bootable on PC, but the iMac gives me a lifeless, black screen. I see grub functioning, and when I try "Try Ubuntu without installing", You can see the usual "APCI probe failed" and another message given to me on PC, but caused no error whatsoever. Then, after that, the screen just turns off. Thats it. When I try "Install Ubuntu" The same things happen, but you can hear the bongoes when the screen goes off. And yes, I have tried the keys that usually start the backsplash and the text showing you what runs in the background. I think the mac is 3 years old.