Robocopying from NAS to External USB 5 Tb of folders containig large and small files

I need to Robocopy from a JBOD NAS to an external USB disk, 5 Tb of folders containig large and small files (movie folders containing the several Gb movie and .nfo, .jpg and .srt small files). Can I run two subsequent instances, one with /J and one without? What does Robocopy whe it finds the folders are already in destination disk? Thks!