You can't copy any disk that contains areas without flux reversals, such as Stormlord (original release).
When you copy such data:
1, drive used for reading transmits no flux reversal at all on the READ signal
2, the drive used for writing keeps the same flux as there is no change on the WRITE line
3, when you read back the data, you will get weak bits instead, as one way of generating weak bits is to exceed a certain amount of time without flux reversals...
That's the idea, it's the ultimate copy-protection, and can only be generated by a commercial duplicator, as you need much higher flux density than what you have with any home-grade equipment.
So why wasn't it used all the time?
On some drives the trick used to generate the data simply does not work... you will have flux reversals read back and the protection will fail even though the disk is original.