I had been using my MacPro (unsupported) on 10.12.2 Sierra and the screen freezes yesterday, mouse and keyboard were not responsive also. So hold down power button and to shut it down. Upon power on, the loading progress bar would be about half and then restart the computer again. Since I have El Capitan on another drive, before i was able to boot up this drive, but this time it will also load half way and restart again. I have tried to use a USB flash drive but it only gone half way and restart. I tried to safe boot by press and hold shift after the boot sound, but it will not go to safe boot at all and then it will load to half way and restart. I tried to press and hold D, same result. I have reset PRAM and SMC, same result. I have successfully press command + s to go into single -user mode, fix disk and it said modified, reboot, same result as it would boot half way and restart again. I am running out of idea.