I want to share my experience with you. My MP arrived few days ago with OS X Yosemite 10.10 on it, and just after setting it up and connecting to network, I installed 10.10.2 update from AppStore and after that Mac Pro has locked up a few times. It was completely random and what happens is GUI just stops responding, and only thing you can do is move cursor around. Sometimes cursor changes in beach ball, other times no, but either way GUI does not respond and you must force shut down....This happened for every few hours... while idling or converting video. And so Googling starts and searching through Console....I have found few similar MP6.1 problems with Mavericks 10.9.2, but fixed with 10.9.3/10.9.4. Every time MP hangs, in Console entry like this appears: Kernel_2015-xx-xx-xxxxxx_Mac-Pro.gpuRestart...so GPU driver or GPU itself. To cut story short, with Mavericks 10.9.5 / Yosemite 10.10.0 installed - no problems for 3+ days. So if you have Mac Pro (Late 2013) be aware of 10.10.2, and of course share if you have/had similar problems.