Personally I'd use RSync, since I'm a big fan. I assume your NAS is probably already hosting a SSH server, so you should just be able to run. You haven't stated which operating system you're running (Assuming Windows due to the NTFS filesystem, but not 100%), but it should work on all the major ones (Windows (Cygwin), Mac, Linux, BSD)
rsync -Prtu /hdd/music backupUser@NASHostnameOrIP:/raid/array/music
rsync -Prtu backupUser@NASHostnameOrIP:/raid/array/music /hdd/music
You'll have to run RSync twice, once for updating your NAS and once for updating your HDD.
The flags mean:-
-P = --partial --progress
--partial = continue previously interrupted transfers, as oppose to restarting
--progress = print progress to the command line
-t, --times = copy the time metadata to the filesystem (So that the system can determine which file is "newer" on your two-way sync)
--recursive = Synchronise all files, including those in subdirectories.
--update = Do not replace/copy files that are newer on the destination
The combination of using
--update on two transfers going in opposite directions means that the end result will be that both origin and destination result in the same file set, those with the newest files.
If you'd like to have automated transfers, the best solution for that would be to utilize private keys for your SSH connection to your NAS, and the scheduling system built into your operating system.
Windows:- PowerShell's WMI Events
Mac OSX:- Launchd's com.apple.device-attach