I have been fighting with this trying to help a friend. The friend bought an iMac A1225 and wasn't aware that it had Windows 10 via BootCamp and reformatted the HD Apple's extended journal with three partitions. He then installed El Capitan and Yosemite via firewire from my laptop, then left the third to install Snow Leopard via the DVD. Well, we could not understand why Snow Leopard DVD would not boot...progress bar stopped halfway. So we attempted to boot El Capitan as well as Yosemite...they both did exactly the same thing where the boot progress bar stopped halfway. So, we tried restoration of the firmware; however, the iMac would not beep when holding the power button. Then, we tried booting rEFIT and discovered there was a protective MBR partition in the first position who's operating system had been removed when we reformatted the HD to the extended journal. Since we can't get terminal on the machine to use the GPT Fdisk (downloaded Fdisk but it's bootable), Would it be easier to just reinstall a Windows OS on that protective MBR partition? I just downloaded Windows 7 Ultimate iso to put on USB as the DVD would not mount. What a nightmare Bootcamp created! BTW, we had tested the iMac with AHT and no problem found. Also, the recovery partition did the same thing...boot to halfway as well.