clusteralloc
int clusteralloc(struct msdosfsmount *pmp, u_long start, u_long count, u_long fillwith, u_long *retcluster, u_long *got);
error = clusteralloc(pmp, cn, count, CLUST_EOFE, &cn, &got);
status = B_FROM_POSIX_ERROR(clusteralloc(fatVolume, 0, 1, CLUST_EOFE, &newCluster, NULL));