WARNING - Google Drive Incompatible with Calibre!
You are currently at significant risk of data loss, please move your library out of Google Drive immediately.
If you want to use Google Drive as a backup, use a tool like Microsoft SyncToy to
mirror a copy of your library into Google Drive. Even when (not if) the folder structure gets borked and the database is thereby rendered invalid, you will still be able to import the books into a fresh library and start anew -- so it can still serve as a backup, but only if you don't mind having to completely re-import your library.