Why does BitTorrent Mainline create or download parts of files I set to “Don Download?
This occurs because BitTorrent has no concept of files, only pieces. Because multiple files can share the same piece, and BitTorrent Mainline has to download an entire piece to check its hash, it will effectively download data for another file (regardless of whether it was skipped). There is nothing you can do about BitTorrent Mainline downloading data for another file if the data is a part of a piece that also belongs to another file that you do want downloaded. What you can do is prevent the entire skipped file from being allocated by enabling diskio.use_partfile.
