I am trying to install Windows on this iMac, early 2009, running El Capitan through boot camp. But each time I try to do so through Boot Camp Assistant, I run into this error, Boot Camp only supports installing Windows 7 on this Mac. please insert a USB or DVD which contains a full version of Windows 7. I am positive I have Windows 7 disc in the superdrive, this is perplexing. Any help is much appreciated. Just to add on, I burnt the Windows 7 ISO via right-clicking on it and selecting "Burn"