How to back up one of the Nas drives onto the other without using as RAID?

I currently have Time Machine using one of the 2TB Hard drives as the backup destination. I use the other 2TB drive as networked storage for photos, music and videos. I’d like to back the 2TB drive with music, pics, and videos to the 2nd 2TB drive that is being used for Time Machine. 

What’s the best way to do this?

I do not know if it is the best way, but for me the easiest way to do it is manually:  How to copy or move files or folders to a different hard drive on a Mac