Thanks Amigaharry2!

Yes, I'm sure that "hd" is properly set because "dir hd:,\\" shows the content of the Boot: volume, and the file "bootinfo.txt" is of type "tbxi" (all the others are "????").

It's very puzzling because starting the G5 "as is" will not start MorphOS but:
- Going into OF and typing "bye" immediately will start MorphOS
- Going into OF and typing "multi-boot" will show the MorphOS "disk" and selecting it will start MorphOS

Crazy, eh?