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The expert can run an md5 hash list containing the signatures of all the copyrighted music that the RIAA has collected over the years and compare the results against the contents of the hard drive. You can name a file anything you want and its content based md5 will stay the same.
That's why you change the file in some way, for example write something random in an MP3 file's ID3 comment tag. The resulting md5 hash is now completely different and most likely is not included in their list.