I assume Volumio stores all the information about the music library in some sort of database file.
Where is this file(s) stored so that I can back it up?
Volumio seems to take forever to index my NAS and I need to re-flash a later version of Volumio and don’t want to have to re-index the NAS from scratch.
There was a discussion about this a while ago, not sure if anything resulted from it.
It would be a nice idea for a plugin though, being able to make a backup of music directories, including the settings for the corresponding NAS/USB drives.
Any volunteers? 
Is there any config-file where you can point the database to an externial usb media device (like minidlna) ?
No, there is no current provision within Volumio to achieve your aims. It would be much easier to achieve from within a plugin (assuming that someone is willing to write one).
Thank you! Good to know, because Volumio with minidlna, Usb hd, bubbleupnp (Android Tablet) is working flawless and I see no advantage to try mpd and maybe mpdroid.
So is the library database stored in a file that can be copied for backup?
Did you ever find the database file location?