Not a rpi trick but Asustor (as1004t v2) NAS sync with external USB drive

Synchronise NAS with an external USB hard disk or key

1. Install entware to install rsync

Log you on NAS ADM

Install entware with "App Central"
In config panel, enable ssh



2. Use rsync

Log you on NAS with a ssh terminal

Install screen

$ opkg install screen

Start a screen

$ screen -S toto

Display the list of files that will be sync

$ rsync -a -v -n /share/USB1/ /share/Media/
...
... the list of file to sync
...

Synchronize files

$ rsync -a -v -P /share/USB1/ /share/Media/

Detach screen

Ctrl a + d
$
...
....
...

Attach the screen to check the progress

$ screen -x toto

Other interesting rsync options :

-u           ignorer certains fichiers plus récents dans le répertoire cible (update only)
--stats    afficher le rapport quand la sauvegarde est terminée