I'm trying to fix an iMac that won't boot any disk.
The Verbose Boot option is on and a few lines are logged, but then it stops. A couple of seconds later, the computer restarts all over again with the bootup chime.
The restart happens so quickly I couldn't read it all, but captured a photo of the display:
I've reset the SMC and the PRAM. Holding down Shift for Safe mode, or Cmd-R for Recovery mode does the same thing.
If I hold down the Option key, I can get to the Startup Manager screen showing the "Macintosh HD" and also a bootable USB (El Capitan Installer) disk. Whichever disk I choose to boot from, the same thing happens.
If I hold down D for the hardware Diagnostic mode, it runs for about 2 minutes and reports "No issues found". From there, it tries to boot into Recovery mode but the same failure occurs.
Have I tried everything?