What is Proof of Music Ownership?
scottsk asks: "What is proof of music ownership? I can't find a good answer anywhere. Let's assume some random person is hauled into court allegedly for having music that he has not legitimately bought. What must that person produce to prove the music was purchased legitimately? Is producing an original commercially sold CD with the music acceptable, or is some further proof of purchase needed (cash register receipt, cancelled check, etc.)? What if a person has digitized a commercial cassette, like digitizing a photo? Must the person carry the cassette around forever, or is just the cassette insert sufficient? (What about an LP record that has been digitized?)" Now, what happens if you've lost all of your property in a fire, but still had an off-site digital backup of your legally purchased music somewhere? Does the loss of the original property invalidate the legality of the backups?
Honestly, I think that the RIAA would try to put a some spin on it (like that you're not allowed to use backups from another person's license or some shit). I don't think this would stop them.
Anyways, to answer the rest of your question: I'd guess you'd need the reciept; how else are they to know that you're the one who bought the cassette or that you didn't buy it after you were charged?
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John Fogarty, who was sued by Saul Zaentz for allegedly plagiarizing himself , might be an even better example.