After trial and error, it's simpler to have a dedicated drive for the file history. The first save can take a long time, so set your power options so they don't turn off your computer until the first save is complete. This is especially relevant if you are using a laptop that you move around without the external drive connected, then you know you have a full backup to start with. It's disconcerting to find that your initial backup isnt complete after several days on the road.