I apparently cannot find an answer in the documentation, however initdb states:
initdb: directory "/mnt/data1" exists but is not empty
It contains a dot-prefixed/invisible file, perhaps due to it being a
mount point.
Using a mount point directly as the data directory is not recommended.
Create a subdirectory under the mount point.
and effectively /mnt/data1 is a mountpoint, therefore but belongs to
the user of the cluster, therefore the only difference I can see is
having '..' belonging to root at the mountpoint or the cluster user
within a subdirectory.
What's wrong with using a mountpoint?
Thanks,
Luca