here is my dirty solution
1. i do a full backup copying different files
2. i run this script
PHP Code:
#!/bin/bash
#this must be run as root !
externaldrivename="ice"
#replace ice with the name of your external hard drive (if it has spaces, put a \ before each space)
for i in Applications Library Developer System usr bin etc private sbin
do
echo "let us check what should be deleted from $i"
rsync -n -rltv --delete /$i/ /Volumes/$externaldrivename/$i/
#do check what is gonna be deleted ; when you're sure, delete the " -n " (which forces rsync to do a dry run)
done
(i volountarily ignored /Users, because i do want to keep files on the /Users folder of my external hard drive that are not on the volume from which i clone)
hope this helps someone !