Well, I was using the following command:
calibre-server --develop --with-library "/mnt/HD_a2/Calibre eBooks" -p 9562 --daemonize
I switched it to using:
calibre-server --develop --with-library /mnt/HD_a2/Calibre\ eBooks/ -p 9562 --daemonize
Everything seems to be working... so I am manually moving books until it breaks (hopefully it won't!!) ... By chance will this be writing to a log file anywhere? This is a headless NAS so I won't actually see the output otherwise...