Well, I have done a lot of "special"
trying to cheat Calibre that its database is located on a local drive but I've apparently failed. The reason of my question was exactly what
itimpi said - for some of forum member it somehow works. I wish I was one of them.
What I most likely do is copy my Calibre collection on a local drive and set up a sync mechanism to have an exact copy on my NAS server for security purposes.