Comment 29 for bug 1758548

filesystems have no concept of atomic operations. You cannot have
multiple software accessing the same set of files at the same time and
expect things to magically work. If you put your calibrelibrary on a NAS
or a folder that is modified by other software, things *will* break.

https://manual.calibre-ebook.com/faq.html#i-am-getting-errors-with-my-calibre-library-on-a-networked-drive-nas