After my last post I tried again and I think this works!
Make these settings under Storage settings:
Network Neighborhood Mode: Share all patitions in disk
FTP Mode: Login to first matched shared node
Create a user list with the username and password you want for FTP.
Enter all you disk partitions under Shared nodes list and edit the all to remove the Guest Share and Write options (uncheck the boxes).
In the partition you want for FTP, select the username from the dropdown list and give them the Share and Write permissions.
Click Apply, Finish and Save&Restart. When the WL-HDD reboots it chucks out all anonymous FTP logins, accepts ones in you User list and allows all ordinary network access.
I wish ASUS had made it a bit clearer.