Thanks darryl.
Understood.
I fail to understand why DB locking seems so difficult (I do routinely access databases, even embedded SQLite, from multiple sources at the same time with no adverse effects) to have a full reentrance, but that's probably just my fault.
As said I do *not* plan to access a full Calibre concurrently at all.
I could also put in place some failsafe to avoid concurrent access "by mistake".
In all cases my full NAS is backed up on another one *and* on a cloud (better safe than sorry
