Also: MIRROR only (Active >> backup ), never 2 way or you can end up with
the sync putting back stuff that was changed.
FWIW The Calibre DB is in lockstep with the Library files and folders. Break the cadence (net/cache delays) and the garbage builds up.

Freefilesync Takes about 2 min to scan for changes and another min to do the copy phase (drive on local wired LAN)