I regularly back up my music collection (which has reached 24.5 GB of mp3 folders) using Robocopy. The collection lives on an internal hard drive (Drive D: under a folder called Music) on my main PC, and the backup is to my NAS drive which is connected to a router (Mapped to drive letter V: with a folder called \Shared\Music). I add new folders to D:\Music and I run this command to make the new folders and files appear in the backup:
robocopy "D:\Music" "V:\Shared\Music" /S /xc /xn /xo
Only the new folders are copied