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.

If you are using Windows Home server then you may experience a different Windows issue. Windows Home Server (WHS) 2011 can automatically read/overwrite the MP3 tags and album art of music files stored in WHS shared Music folder. This solution from Microsoft may help to resolve the issue:

1. On Windows Home Server, open Task Scheduler by clicking Start > Administrative Tools > Task Scheduler
2. Navigate to location \Microsoft\Windows\Windows Server
3. Right click 'Media Streaming Metadata Synchronizer' and click Disable

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.