Hi trying to upgrade to the latest beta on my 2015 iMac Retina display. The system reboots and freezes about 1/3 of the way through gets stuck at "extimating time reamaining". have tried force quitting / rebooting usnign keystrokes ("SHIFT" or "COMMAND+OPTION+P+R" or "SHIFT+R" or "SHIFT+D") but none are registering.
My next step will be to restart (or try) in external drive mode and dig around...
Does anyone out there have any better ideas?
Thanks for you help.