[kovidgoyal]

calibre now locks folders when copying book folders not just files. This was done because some people were getting errors caused by other programs locking folders and thereby causing the copy operation to fail halfway through.

This means you need to close windows explorer/not keep it open on a calibre library folder if you intend to change the title/author of a book. This entire issue is largely because windows file system semantics are absolutely bonkers. It is impossible for multiple unrelated programs in windows to share access to the same folder/files robustly.