how do you know that your data is stored locally?. google documentation is not good and google do tend to change how stuff works without warning.. this discussion suggests you may be right & I may be wrong though:
https://productforums.google.com/for...ve/NFMVzlwdE3c
there a different rules for "google documents" apparaently, I let those confuse me.
Because the Docs files in your Drive folder are essentially links to files that exist online,
That is specifically about Google Documents, not normal files. Uploaded files are synchronised intact, and stored in both places. Google Docs are synchronised as links (unless you enable "offline", in which case it is a little more complicated, but still not the same as sync).
I suppose I could test by moving stuff in/out & monitoring what windows says about disc usage, but that's by the by.
I accept what you say about it not being a reliable a backup as a simple local copy would be.
But it could be that the bug you found no longer exists, because google have tweaked their code.
PS to humour you on the testing, I checked my Laurel K Hamilton author folder, which has ~40 book folders. All the 4 digit numbers match. So I reassert that the process of having windows write a copy of an entire calibre library to google drive, synced to one machine only, and while calibre is inactive. is a safe one-way procedure. remove any of those pre-conditions and yes, it may no longer be safe. Hence the suggestion of more collective testing & documentation.