I’ll make a long story short.
After I was dissatisfied by OS 5 on my WDMC Mirror Gen2 I was installing DSM 6 on the device based on this thread:
But after a few days it crashed.
So I managed to activate the original WD rescue and installed OS 3 again.
From there I installed Debian Stretch based on this:
followed by OMV 4.
By accident I installed Debian 10 (Buster), but I was unable to install OMV then.
If you do the same, take care and modify the sources to use “oldstable” which is Debian 9.
Otherwise you will sooner or later upgrade to Debian 10 maybe by accident.
To work around a dependency error with NGINX while installing OMV do:
edit /etc/nginx/sites-enabled/default and comment:
#listen [::]:80 default_server;