we will use rsync to do this (works with mounted USB drives as well)

sudo apt-get install rsync

now we automize it:

create sync script:

nano ~/sync_music.sh

rsync -av --progress /mnt/NAS-music/ /mnt/music/

dont´forget to make it executable

chmod +x sync_music.sh

This script might run for a very long time… To make it independent from your session you may use a terminal multiplexer called tmux. You find more details at the endo of the following article:
10 Maintenance – Hauptsache es klingt ! (apfelbeck.at)