Be verrrrry careful.
The same DB/Library WILL turn into a mess if you use any part of Calibre (except server) to make, even tiny, changes from the
OTHER OS.
Been there, the scars were horrid
safest
1) (
USE the CLI) Content server viewed with you browser of choice from remote machines (or when using the alternate boot)
2) drop books to be added into the Auto add folder
in the Host OS of the GUI Calibre. They will be added (shortly) when that GUI next starts