Calibre does not take libraries on network drives. It will throw "Database locked" errors when you try.
But we can avoid this error by mounting the volume with
From the manual:
Do not send byte range lock requests to the server. This is necessary for certain applications that break with cifs style mandatory byte range locks (and most cifs servers do not yet support requesting advisory byte range locks).
So in the
//192.168.xxx.xxx/nas /home/username/nas cifs nobrl,username=username,password=xxxx,uid=1001,gid=1001 0 0
Then you can use the calibre libraries on this volume without errors.