Photos on external disk are not recognised

My situation is as follows:
Photostructure for server (Node) installed on a Ubuntu server with username and groupname: caddy. Webserver is Caddy. In the Caddyfile I put a reverse_proxy to localhost:1787. In the Systemctl file of Photostructure I gave in the [service] header the username and groupname= caddy. Otherwise the Caddy server with Photostructure does not work. My library is in a subdirectory the photostructure-for-servers directory (user: caddy). I choose to scan for photos manual. My photos are in /media/pi/photos (external disk). The owner of the photos is pi. Photostructure does not recognise the photos in /media/pi/photos. When I copy the photos from external disk to a server directory with owner pi Photostructure does recognise the photos.
I don’ t think it is a problem of user-rights. Or does it?
In this forum I saw somebody with the same problem but no solution.

PhotoStructure doesn’t need write access to files in order to import, but it does need a way to discriminate between mount points, or else it has to resort to saving asset file variation records with a file: URI.

I wrote up why here:

I suspect if you do this step, things should work:

It works, thanks. Stupid that I didn’ t see this.

Not stupid at all: I don’t know if any other software that does this sort of stable file URI generation, so it’s not going to be intuitive.

I need to add a “missing volume UUID” message to the health checks, perhaps?