My dad had his own business selling copy protection he invented for 5.25" floppy disks back in the early 80's. It didn't use codes from the manual though it us

My dad had his own business selling copy protection he invented for 5.25" floppy disks back in the early 80's. It didn't use codes from the manual though it used some trick with the floppy where a special sector could be read but wasn't written under normal conditions when the floppy was copied.