I know this topic has been discussed and there are guides, but I've followed them exhaustively and I'm still at a loss. I have DP2 installed on my iMac. I want to buy Lion Retail. I've tried downgrading to Snow Leopard but I can't! One SL DVD won't load, the other loads but says 'Mac OS X cannot be installed on this computer'. I've erased the volume. I've tried partitioning and installing. I still can't downgrade. Now I know someone's going to say 'erase your entire hard drive and GUID format it'. But my Windows partition is going to be a bitch to restore... please don't make me do that! Any ideas would be greatly appreciated. UPDATE: Just tried creating a separate partition and installing SL on that. 'Mac OS X cannot be installed on this computer'. Does that mean it's a problem with the Firmware?