When I try to boot using my flashdrive that contains Ubuntu, I hold down the option button during startup which brings me to the page where I can choose where to boot from. The only problem is that the flashdrive won't come up. I accepted this and tried to partition my hard drive to do it that way but discovered it won't let me. So I'm back to trying to get my IMac to boot from the flashdrive. By the way I'm running OS 10.4.11 and trying to install 12.04. Thanks in advance!