Crazy stuff. Was reorganizing all of the wires around my desk to clean it all up. Plugged everything back in, power my MP on and then plugged my powered usb hub's (which I have used for several years) USB cable into the Mac Pro, suddenly, it shut off and started making a clicking sound, similar to the one it makes when you first power it on but over and over again. I was like well that's very odd. I unplugged the MP, then plugged it back in and it would not turn on, just started making the clicking sound as soon as I plugged the power cable back in. I was like well that's really weird. I unplugged my USB cable from my hub and the clicking stopped and I was able to restart it normally. Plugged it back in and it immediately shut off and went back to clicking. I went to de-power the hub to test that, and wouldn't you know it, in moving everything around, the power cable to the USB hub came out most of the way and was making poor contact inside of the hub. I secured it, plugged into the MP and everything has been working normally since. Is there some kind of short/over-current protection in the 4,1->5,1 in the USB ports? Tried a few of them during troubleshooting and all the same result. I think it may have just saved this thing. If so, I am glad I plugged this Mac in before my gaming PC, as it may not have such a feature. That would have blown (no-pun intended).