I had some issues when upgrading RAM in 2 Mac Pro's. In my own Mac Pro I installed 32 Gb to replace the the current 16 Gb. I had to do some restarts because the Mac hung on a total 'white' screen: the Apple logo became not visible. I could resolve it by booting from the Snow Leopard DVD and choosing the correct boot drive. After that all is fine. Then I installed the 16 Gb in the Mac Pro of a relative who has Moutain Lion. Strangely enough in the start up the Apple logo appeared but then a progress bar appeared. After some minutes the progress bar stopped and the machine turned itself off. This relative also uses Bootcamp and we did not have any issue booting with Win7 (installed on a different drive). I'm confused by what's going on. The only thing I can think of is that Mac OSX and especially Mountain Lion is very picky when it comes to installing new memory??