So I had the first Sierra public beta working on my Mac Pro (4,1; firmware upgraded to 5,1) with an Nvidia GTX750ti connected to a 4K (UHD) Samsung display. I had it installed on its own internal SSD. When the 2nd public beta was announced this afternoon I booted into Sierra to turn off the Nvidia web driver to prepare to do the upgrade. But after doing that, Sierra will not boot. The progress bar under the Apple logo gets to about 60% then it just restarts and I get the "your computer crashed" message. Booting to the Sierra restore drive does work. A safe boot also crashes. I did have a similar problem when I before I got the first public beta working. In the end I had to disable the Nvidia web driver in my El Capitan install, which for some reason allowed Sierra to boot. But now that trick is not working now and I'm out of ideas. I've tried: Resetting VRAM Resetting SMC Re-installing Sierra public beta 1 But nothing has worked. Does anyone have other suggestions for me to try? It certainly feels like a graphics driver issue to me.