sdutil cleanup start_part_dos (), write_iso.sh only work with block devs

aux/write_iso.sh

@@ -3,6 +3,17 @@
 # Requires sudo
 # USAGE: ./aux/write_iso.sh /dev/sda
 
+TARGET="$1"
+
+if [ -z "$TARGET" ]; then
+  exit 1
+fi
+
+if [ ! -b "$TARGET" ]; then
+  echo "Not a block device"
+  exit 1
+fi
+
 set -x
 
-dd if=./mop3.iso of="$1" bs=4M status=progress oflag=direct conv=notrunc,sync && sync
+dd if=./mop3.iso of="$TARGET" bs=4M status=progress oflag=direct conv=notrunc,sync && sync