My Book LIve safe point to MyNet N750 USB drive misbehaving

So I picked a 1 TB WD My Book LIve NAS and WD MyNet N750 wifi router. Connected a USB drive to the router and setup a safe point creation from NAS to the USB on the router. While it took a few faild attempts to get it done but finally it was done. The problem I am facing is that whenever I do update of the safe point, the process starts afresh and copies all files again taking a full 11-12 hours for the 220 GB data on NAS. It is true even when I did not change a single file on the NAS. Is this expected behaviour or am I doing something wrong? Any things I can try out to figure out why it is happening?

when you are setting the safepoint, you will see an option that says auto update, if you enable that, it should do the incremental backup.

Thanks. Will give it a shot but are you saying, incremental works only with auto update and not manual update ?