I still got a Mid 2007 iMac laying around as a spare mac. It's still running fine infact it is running Yosemite but that is on a external USB 3.0 Harddrive and not the internal HD. if i boot it from the internal harddrive which had the OS that it was shipped with it just loops and loops in this whitish/greyish blank screen after the charm sound. On Yosemite running from the external harddrive i can see the harddrive and i can still access the files on it but why isn't it booting from it? Took it to Apple back before i ran it off an external harddrive and they said the internal harddrive is dead and it will cost $$$ to replace.