I was about do a clean install of 10.8 on my early 2011 15" MBP, but since 10.9 DP came out, I did an upgrade to try it out before wiping everything. Turns out I can't downgrade now... When loading the installer from a USB stick, the logo above the spinning wheel changes into a prohibitive sign (slashed circle) after a while, and it never loads. I tried running the installer from the hard drive just to see if it runs, but got the "software cannot be installed on this computer" error. So I wiped my hard drive, but I still keep getting the prohibitive sign. Any ideas why this could be happening? Could the 10.9 installer have changed the EFI somehow? Is there any way to force the 10.8 installer to load? I'm currently stuck with a largely non-functional system, as some critical apps don't run or keep crashing on 10.9... EDIT: By downgrading I mean doing a clean install, not installing over the existing system.