As an Apple developer downloaded the installer, and it restarted and started installing. After about 30 minutes it said it could not install MacOS and said needed to restart. Then got an error in mutliple langueges and in the top left showed some terminal code, and kept starting and restarting about 4 times in a loop until showing a circle with a dash thorugh it (no smoking sign), and then went black. Same issue if manually restart. No way to get out of this. Have a wireless keyboard so will so if I can wire up with thunderbolt cable to get into safe mode to reinstall Sierra, otherwise Eek!
This is on a new iMac 5K that was upgrading from the latest dev version of Sierra.
