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davidfor - and I might have misread a different part of the opening post
I thought it said the calibre-server was being stopped before starting the MAC client. But on re-reading it would appear the OP leaves calibre-server running while updating the library from the MAC, then when the MAC is closed the server is stopped and started. My guess is that this sequence is used to minimise the server unavailability time.
Would calibre-server.exe running on the server against a library on a local drive detect that a MAC elsewhere on the network was running the calibre GUI against the same library - or vice versa? I am under the impression it wouldn't, but I'm not up to speed with the version 3 server - I use the old 2.85.1 server.
I recall reading of user implemented schemes where the presence of a 'library.lock' file in the server hosted library folder was used to prevent concurrent execution of calibre.exe on two or more workstations.
BR