In reply to:

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If you insert a id3v2 tag, or when its size increases, the mp3 frames are shifted to a different location in the file. As far as I can tell, rsync cannot detect this and compares the whole file, only to find out it has to transmit everything again, because none of the file parts are identical.

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If that happens, your rsync isn't working correctly. The rsync protocol is designed to find common parts of files so that it can cope with this kind of thing - otherwise it would also fail on e.g. tar files when a file is added near the beginning. (Of course, compressed tar files are a different matter...)