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?