You are on the right track, but the volume name is wrong (at least for me). The system mounts external USB drives as "/volumeUSB1/" or "/volumeUSB2" (only two USB ports on my DS212J). I found the correct volume name by looking at Shared Folder-> Edit -> NFS Permissions for the "usbshare". It showed "Mount Path:/volumeUSB1/usbshare" in grey on the bottom of the window. If your mount is different you should be able to find it using Disk Station Manager on that same page. I changed my sync.conf (below) to insert the new volumes. And I can confirm this works. You MUST set read/write permissions for the share for your admin user also. So thanks for the pointer and I hope this gives everyone the complete solution.