Hi All Newbie here. I fitted a "working" HD 6970m which I got off eBay. I can get it to show the initial boot screen but then the screen goes goes blank/white. I then tried booting and holding the "Alt" key and successfully get the different drive options. I can choose which ever. I can boot into windows (I hear the chime when I press the power button and the machine powers down). If I try any of the other drives including a Yosemite install USB eventually the boot fails and sometimes gives "boot disk not found" message. Does this mean the card is busted as per the latest Apple card swop deal on these or is there something else a-foot that I'm missing? I am running the Yosemite beta and machine is an i7. Thanks for any help.