The Problems With Game Copy Protection
The Apple II was (and still is) also good to produce copy-proteced disks for computers with a floppy controller of the Western Digital family. Format a track with the WD controller, put the floppy into an Apple, write the same track with the Apple "controller" (it is actually just a shift register) for a brief moment and interrupt the procedure so that only a small fraction of the track gets overwritten. The Apple does not care about the index hole and starts to write at a random location. With a bit of luck you overwrite just a fraction of a sector written by the WD controller. Repeat until you succeed. The Apple writes the data with a different clock frequency than the WD controller. Thus every time you read the prepared disk with a WD controller, the WD controller's pll oscillator fails to synchronize with the bits of the manipulated sector on the floppy. Consequently the controller produces random information, plus a checksum error, every time you try to read the manipulated sector, very much like a hole punched into the floppy disk. From the outside the disks looks perfectly fine. And there is no way to duplicate that without a lot of effort, certainly not with a standard controller. This scheme still works today for PCs. The PCs floppy controller is just a clone of a NEC controller, which itself is a clone of the WD controller.