Back in the Apple ][+ days there was a copy protection scheme that depended on the spindle speed/timing of the old Apple ][ disk drive. The original would be recorded slightly off the proper speed, but still readable (unless the timing was off the other way on your drive unit). Copies would record at normal and the software could detect that and refuse to run. An issue of the old Hardcore Computist magazine (core/apple pun intended) had instructions for adding a speed control knob to a disc ][ drive unit so you didn't have to take the cover off and use a jewelers screwdriver to adjust the potentiometer. In college I occasionally made a couple extra bucks teaching Apple software classes at computer stores. One customer had an issue with a drive unit that was spinning to fast being unable to read the software. A little bit of screwdriver work fixed it right up.

edit: Found some old digitized library of issues online. Not sure which issue it was: http://computist.textfiles.com/