diff --git a/scripts/combined-ext-image.sh b/scripts/combined-ext-image.sh
index 0c7b1d993d..7ff287bde7 100755
--- a/scripts/combined-ext-image.sh
+++ b/scripts/combined-ext-image.sh
@@ -22,6 +22,8 @@
 ## version history
 # * version 1: initial file format with num files / name / length / md5 checksum 
 
+set -e
+
 ME="${0##*/}"
 
 usage() {
@@ -38,7 +40,21 @@ IMG_OUT=$1; shift
 FILE_NUM=$(($# / 2))
 FILES=""
 
-printf "CE%02x%-32s%02x" $CE_VERSION "$IMG_TYPE" $FILE_NUM > $IMG_OUT
+tmpdir="$( mktemp -d 2> /dev/null )"
+if [ -z "$tmpdir" ]; then
+	# try OSX signature
+	tmpdir="$( mktemp -t 'ubitmp' -d )"
+fi
+
+if [ -z "$tmpdir" ]; then
+	exit 1
+fi
+
+trap "rm -rf $tmpdir" EXIT
+
+IMG_TMP_OUT="${tmpdir}/out"
+
+printf "CE%02x%-32s%02x" $CE_VERSION "$IMG_TYPE" $FILE_NUM > "${IMG_TMP_OUT}"
 
 while [ "$#" -gt 1 ]
    do
@@ -48,14 +64,15 @@ while [ "$#" -gt 1 ]
       [ ! -f "$file" ] && echo "$ME: Not a valid file: $file" && usage
       FILES="$FILES $file"
       md5=$(mkhash md5 "$file")
-      printf "%-32s%08x%32s" "$filename" $(stat -c "%s" "$file") "${md5%% *}" >> $IMG_OUT
+      printf "%-32s%08x%32s" "$filename" $(stat -c "%s" "$file") "${md5%% *}" >> "${IMG_TMP_OUT}"
       shift 2
    done
 
 [ "$#" -eq 1 ] && echo "$ME: Filename not specified: $1" && usage
 
-mv $IMG_OUT $IMG_OUT.tmp
-dd if="$IMG_OUT.tmp" of="$IMG_OUT" bs=65536 conv=sync 2>/dev/null
-rm $IMG_OUT.tmp
+mv "${IMG_TMP_OUT}" "${IMG_TMP_OUT}".tmp
+dd if="${IMG_TMP_OUT}.tmp" of="${IMG_TMP_OUT}" bs=65536 conv=sync 2>/dev/null
+rm "${IMG_TMP_OUT}".tmp
 
-cat $FILES >> $IMG_OUT
+cat $FILES >> "${IMG_TMP_OUT}"
+cp "${IMG_TMP_OUT}" "${IMG_OUT}"