I use MS Synctoy 2.0 to sync files on an external drive. You can configure it through task manager to run as often as you want at time intervals...like every hour or whatever. You can also manually run the sync when you are done each time. This is very easy to use and configure and it takes almost no time if the changes are minimal.
http://www.microsoft.com/downloads/details.aspx?familyid=C26EFA36-98E0-4EE9-A7C5-98D0592D8C52&displaylang=en