In case anyone else would want to do the same, I've finally found out that in order to boot from an USB disk, the loading of modules must be enabled, i.e.:
installboot -o modules /dev/rsd0a /usr/mdec/bootxx_ffsv2 Vincent
In case anyone else would want to do the same, I've finally found out that in order to boot from an USB disk, the loading of modules must be enabled, i.e.:
installboot -o modules /dev/rsd0a /usr/mdec/bootxx_ffsv2 Vincent