I have new late 2012 iMac and wanting to make a bootable USB stick I attempted to download a copy of ML 10.8.2 from the App Store. When I clicked on download I received the following message: We could not complete your purchase. OS X Mountain Lion is not compatible with this computer. What's going on? Incidentally I was able to download ML 10.8.2 Supplemental Update but that is not the same thing.