@koszer: yeah, I think I'll wait a bit in order to borrow an external drive to continue

@jacadcaps: initially, the usb sticked was formatted as HFS (and not HFS+) from the installed Macos of this computer. Then I have tried to repartion/reformat it from my Linux computer (one time in fat32, one time in hfs) with gparted and then I obtain other issues (open firmware list no file).
I think I will abandon this usb procedure until I find a drive.

Note that I have also tested to boot from the second harddrive (cf. instructions here http://www.meta-morphos.org/faq.php?id_cat=2&myfaq=yes&categories=Installation+et+d%E9marrage#41), and I obtain similar results (display of CPU information, then stop). If I remember well, the command I launched looked that :

boot ultra1:,\boot.img rd bootimage="HDmorphos:morphos-3.0.iso"

Am I specifying the iso file properly ? (the label of this drive is morphos)