@zab0
What NAS are you using. If it's a Synology I might try myself and hopefully be able to help you.
I sync my calibre ebook files between my desktop (with Calibre) and a Linux server/NAS (with COPS) with
rsync.
If your desktop is Windows you can try DeltaCopy:
http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp
I use the multiple library functionality of COPS.
- one (or more) "large" calibre-libraries with "archived" ebooks/pdf's/comics - the libraries hardly ever change. (no need to sync regularly)
- one smaller library with new (recently acquired) ebooks/pdf's/comics I plan to read or check out if I will really read them. (can be synced or copied regularly from desktop to NAS)
- once every n-months I do a cleanup of the smaller library and use Calibre to incorporate the remaining books in the larger archive libraries. I then make a fresh copy to the NAS.