@
robcohen - you need to read this ==>>
I am getting errors with my calibre library on a networked drive/NAS?
The bottom line is if you run calibre over a 'network' it wont work reliably, services like OneDrive, DropBox, Copy.com etc provide a 'network'
The solution is simple stop the cloudservice before you start calibre and restart it when you're done with calibre - but don't run calibre and any of the cloud service at the same time. And be especially careful with GDrive - there's a sticky at the top of this sub forum
I have calibre working this way with Windows 8.1 on a couple of libraries
BR