diff --git a/tools/tools/nanobsd/defaults.sh b/tools/tools/nanobsd/defaults.sh
--- a/tools/tools/nanobsd/defaults.sh
+++ b/tools/tools/nanobsd/defaults.sh
@@ -716,6 +716,55 @@
 	nano_umount ${mnt}
 )
 
+_populate_part ( ) (
+	local dir fs lbl metalog size type
+	type=$1
+	fs=$2
+	dir=$3
+	lbl=$4
+	size=$5
+	metalog=$6
+
+	echo "Creating ${fs}"
+
+	# Use the directory provided, otherwise create an empty one temporarily.
+	if [ -n "${dir}" ] && [ -d "${dir}" ]; then
+		echo "Populating ${lbl} from ${dir}"
+	else
+		if [ "${type}" = "cfg" ]; then
+			dir=$(mktemp -d -p "${NANO_OBJ}" -t "${type}")
+			trap "rm -rf ${dir}" 1 2 15 EXIT
+		fi
+	fi
+
+	if [ -d "${dir}" ]; then
+		# If there is no metalog, create one using the default
+		# NANO_DEF_UNAME and NANO_DEF_GNAME for all entries in the spec.
+		if [ -z "${metalog}" ]; then
+			metalog=$(mktemp -p "${NANO_OBJ}" -t "${type}")
+			save_traps=$(trap)
+			if [ -n "${save_traps}" ]; then
+				trap "rm -rf ${dir}; rm -f ${metalog}" 1 2 15 EXIT
+			else
+				trap "rm -f ${metalog}" 1 2 15 EXIT
+			fi
+			echo "/set type=dir uname=${NANO_DEF_UNAME}" \
+			    "gname=${NANO_DEF_GNAME} mode=0755" > "${metalog}"
+			echo ". type=dir uname=${NANO_DEF_UNAME}" \
+			    "gname=${NANO_DEF_GNAME} mode=0755" >> "${metalog}"
+			(
+				cd "${dir}"
+				mtree -bc -k flags,gid,gname,link,mode,uid,uname |
+				    mtree -C | tail -n +2 |
+				    sed 's/uid=[[:digit:]]*/uname=root/g' |
+				    sed 's/gid=[[:digit:]]*/gname=wheel/g' >> "${metalog}"
+			)
+		fi
+
+		nano_makefs "-DxZ ${NANO_MAKEFS}" "${metalog}" "${size}" "${fs}" "${dir}"
+	fi
+)
+
 populate_cfg_slice ( ) (
 	populate_slice "$1" "$2" "$3" "$4"
 )