Wow that is a lot of titles. AFAIK the data is written after each book but I could be wrong
I stopped it once midway and the data was written for all the titles.
Perhaps it had just stalled on the last one or something.
I no longer use this method as I found that it would often pick an omnibus or collection of books instead of the original title so I had to hand check anyway and with the wrong titles inserted this was more trouble than doing 50 or so at a time by using edit metadata individually.
Lucky for me I only had about 7000 ebooks and I manage to get through a hundred most days.
Unfortunately a lot of time is taken up by checking readability and removing headers page numbers etc.
Don't know how it could be done without Calibre