File Synchronization Across Local Drives

My Drobo is plenty fast at streaming video until multiple videos attempt to be streamed. I have another external that has the speeds needed to handle multiple streams, but it doesn't have redundancy offered by the Drobo. Therefore, I would like to keep a copy of my videos on my other external that will be synchronized with my Drobo daily and manually.

I don't suppose there is a free application which can do this, is there?