Ensure there is a file called ".ocdata" in the root of the data directory. snap config

This is keeping me busy for some days now.
I installed ubuntu 16.4 in a vm. OK
installed nexcloud 12.05 with snap OK
upgraded to NC 13 with sudo snap refresh nextcloud --channel=13/candidate OK
I login with admin OK
As my vm is small I want to connect external usb drive
mount my drive in /media/usb OK
Did sudo snap connect nextcloud:removable-media looks ok
Followed wiki and changed /var/snap/nextcloud/current/nextcloud/config/config.php datadirectory with /media/usb
Disable after that the mv command as per wiki enable
if I look into the /media/usb directory data I have the .ocdata file…
With snap interfaces I see that :removable-media is nextcloud…

Who can give me some help?

Thanks in advance