Startup Disk didn't recognise the internal drive as bootable. Installing High Sierra beta on the external drive now and will try reinstalling the beta to the internal drive from Recovery Mode.
Here's what happened:
- installed beta 2
- came back to find a not blessed error message and a restart button (not clickable due to not connecting to BT mouse/keyboard)
- force restarted, same screen with different error
- reboot in recovery mode
- ran disk utility first aid, couldn't complete due to unknown error
- ran reinstall macOS beta from recovery mode
- came back to find the dreaded question mark folder
- force restarted into recovery mode, got internet recovery instead (uh oh)
- installed Yosemite on external drive, rebooted, internal drive not recognised (expected)
- upgraded to Sierra on external drive, internal drive recognised (phew)
- deleted the macOS install payload (well tried to, SIP stopped me from emptying trash)
- ran the macOS beta install from Applications
- could not select internal drive because not HFS+ formatted (WAT!)
- upgrading external drive to High Sierra b1
- upgrading external drive to High Sierra b2
- Run first aid on internal drive again (that failed earlier) <- Current status