Rapidlok
One of the toughest copy protections on the C64 — custom GCR formatting on the disk plus a fast loader, built so an ordinary nibbler would fail.
Rapidlok, developed by Datasoft, combined two things: a fast loader and a copy protection that wrote the diskette in custom GCR formatting with extra tracks, half tracks, weak bits and its own sync patterns. A standard 1541 copier or nibbler couldn't reproduce it, and the scheme came in several generations (Rapidlok 1 through 7) that tightened the grip with each version.
Copying a Rapidlok disk required parameter copiers that knew exactly how that particular version looked. The race between protectors and crackers was one of the C64 scene's most drawn-out — and a reminder that copy protection rarely wins in the long run.