Why do my MP3 files keep getting copied even though I've not changed them?
Some media players, e.g. Microsoft Windows Media Player, may modify the last modification date & time of files or the MP3 ID information in the file whenever the file is played.
In order to prevent changes to the last modified date, open Windows Media Player, go to Tools/Options from the menu, and click the Library tab. Uncheck the box next to Retrieve additional information from the Internet. The downside to this is the software will no longer automatically look for album info about the files each time the song is played.
And/or, you may be experiencing this different Windows issue if you are using Windows Home Server (Note: page contains advertising: we are not responsible for content)
Other media players (and maybe indexing software) may exhibit similar behavior. You would need to consult the documentation and/or the authors for a solution.