The cleverest floppy disc protection ever? Western Security Ltd.
We've reviewed the most powerful BBC Micro model B disc protection scheme I found, across an audit of most of the copy protected discs released for the machine. It's clever in that you don't need specialized hardware to create the disc, or read the disc. But you're going to struggle to duplicate the disc.
Copy protection schemes from the '80 and early '90s are fascinating, and this one is no exception.