more poking around does bring up some suggested workarounds, where you park almost all of the library out on a network and do cute stuff with sym links and env. variables.
but it the author's own FAQ( near the end) it explicitly says do not run calibre on a NAS.
( he's much too polite to say "and don't come crying to me if you do, and it all goes pear shaped" but that's sort of implied
)
feel free to google away and also search the forum if you want to get deeper into the technicalities of why it goes wrong and what
may alleviate it. Personally i would not risk it, but my own modest library fits Ok on my SSD C: Drive, so I don't need to