I succeeded using the quick & dirty way: 1. Writing the .hdimg file to a blank USB stick (via dd, see my first comment) 2. replacing the boot.img on my MorphOS 3.18 system with the new 3.20 boot.img from the stick 3. reboot the Mac Mini with the USB stick plugged in 4. Then the welcome screen appears (without iPrefs error message, because the kernal version matches) and you can start the installer and choose a new install
I know this isn't the recommended way to install MorphOS, but booting from the stick didn't work, whatever I tried and this method worked like a charm.
