Back in the late 80's I had a Tandy Coco 1 that had a copy protected game called Dallas Quest. I spent some time copying all the sectors from the master to the copy, and it always got stuck on this one sector. I guessed that the game would check if that sector is readable before deciding it's an authentic disk. I tested that idea by writing a basic program to continually write to that sector, and I would randomly open the disk door to try to corrupt the write. Eventually it worked, I corrupted that sector, and it was unreadable. The game was able to run now on the copy.