As my previous message got moderated, just a quick summary - looks like a GEOM_PART silently discards writes to MBR with a DD when it considers MBR broken (writing to the rest of the disk is possible). I have reported this as error: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=242341 -- CeDeROM, SQ7MHZ, http://www.tomek.cedro.info