I found this post: http://sphardy.com/web/readynas/how-to-migrate-to-user-security-mode/
and I applied it to the USB HDD (bottom of the Shares/Share Listing screen - requires scroll down!!)
but no matter how many times I apply the changes on that screen, it remains the same...
Share folder owner :
Share folder group: avahi
ps. when I tried to backup the USB HDD on the NAS using Frontiew, it failed for the same reason I guess.