MacPro 3,1 2 x 2.8GHz Quad Core Intel Xeon running 10.9.5 What happened, so I was moving RAM from one old MacPro to another, then when I put it back the machine wouldn't boot. I put the RAM in a 1,1 running 10.9.5 and it worked fine, the same RAM that went into the 1,1 went into another 3,1 and worked fine. What happens, starts with a 'boing' and gets a grey screen and a loading wheel, and looks alright. But after a min or so(it takes a long while to boot when it is working) the screen slides in the create a square, it goes from grey to white and the resolution on the loading wheel goes way down(and it gets bigger). It then is stuck here with a couple second loop where the screen flashes black and back to the white. https://goo.gl/photos/8EzhP9eifuyDqKPs6 (video) What I've tried, moving the RAM trays around, reseating EVERYTHING RAM related, video card and junk hasn't moved. Removing each set of RAM individually. Resetting PRAM. Trying to boot off a 'protogo' USB recovery stick, I was able to choose the startup disk but it went the same way. Everything had the same results. I'm now backing up in target mode and am planning on reinstalling the OS. I'm all ears for other things to try, ideas on what might have caused this, thoughts on what this is. thanks so much.