My friend just tried installing Sierra on a blank internal hard drive in an iMac 21" 2010 (model iMac 11,2), using a bootable installer on an external USB drive.
Seemed to install successfully, then rebooted, with a message saying the install had failed and advising: Shut down, turn on power, hold down
D for Diagnostics.
D does not actually activate any diagnostics.
Any idea what might be going wrong, or what to try next?
When trying to run in Safe Mode, this screen appears.