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At this point boot.img has been successfully loaded and the problem is the next step where morphos.iso should be loaded/booted. So, the USB stick seems to work, because at least something can be read from it.

Double check that you have matching name for the morphos.iso on the command line you write and on the USB stick. "morphos.iso" vs "morphos-3.10.iso" etc.

Then try to boot into bootmenu, if you can see it. Add "bm" option to the end of your boot line.