give_sd_to_plex
sdindex = give_sd_to_plex(sd->plexno, sdno); /* now tell the plex that it has this sd */
give_sd_to_plex(plexno, sdno); /* and insert it there */
int give_sd_to_plex(int plexno, int sdno);
give_sd_to_plex(plex->plexno, sd->sdno); /* and give it to the plex */