No, the clue’s in the name It’s a one-off single process, just triggered by the drive connection rather than by manual starting via the dashboard.
When you connect the drive, it will trigger the backup, which will then proceed to the end and stop just as if you had manually started the backup. But at the end, if you want to backup again, you have to either manually restart it or disconnect and reconnect the drive.
As it stands today, there is no “auto sync” option within the dashboard between the built-in drives and USB connected ones. It can be set up using SSH and a cron job, but that will be messing aroud with the NAS at the base command line level and will probably invalidate your warranty if things go wrong.
Have a look through the suggestions thread as this kind of question (sync’ing in general, not specifically NAS<>USB) comes up quite regularly and has probably been made there, where you can support it and comment yourself.