hd:,\\:tbxi would suggest a mistake in the command line you've typed. Try a stick no bigger than 32GB, FAT32/MBR formatted. Copy the ISO file to the stick, name it morphos.iso. Mount the ISO file and copy mac_ppc64/boot.img to the stick as well. Then in OpenFirmware: ud:,\boot.img bi umsd0:morphos.iso must work. Do make sure that :,\ are typed in correctly if yours isn't a USA keyboard!