I just got a mid 2011 Mac Mini i5/2.5GHz with AMD 6630m 256MB video graphics. I am trying to install an OS, but no matter what I do: It boots as usual with gray screen with the Apple logo and progress bar BUT about 1/2 way through the screen goes black for a split second and then the same gray screen appears with nothing else and seems to be stuck there. I used a new Sierra USB and even tried NetBoot but (like I said above) it always boots as normal but aprox. 1/2 through the the screens goes black for a split second and then it's stuck at a blank gray screen. Any Ideas???????