Gather 'round, children, as I tell you the tale of Protect-O-Disk. 
Protect-O-Disk is ©1981 Bill Basham. Bill later got out of the copy protection business and wrote and sold a number of titles under the "Diversi" brand, including Diversi-DOS, Diversi-Copy (unprotected disks only!), Diversi-Dial, and the 16-bit Diversi-Tune.
The idea behind Protect-O-Disk is simple: floppies can fail, and legitimate software owners deserve the right to make backups. But whither piracy? As a compromise, Protect-O-Disk allows you to make a "dormant" backup that will not boot but can be activated from a damaged master.
Protect-O-Disk was used on (at least) three different games from IDSI: Juggler, Shuffleboard, and Trick Shot. @option8 was kind enough to provide me an original disk of Juggler.
Also, apparently, The Professional Blackjack Simulator by Empire Software, according to this 1982 ad in Softalk Magazine: archive.org/stream/softalk…
As shown in the first screenshot, Protect-O-Disk announces itself on boot. Pressing "P" takes you to its main menu, which displays 5 options – make a dormant copy, make a copy with 2 drives, activate a dormant copy, more help, or continue booting – and your disk's serial number.
The instructions explain the system: you can have many "dormant" copies but only one "active" copy.
Then it explains the restoration process. In case your "active" disk is damaged (but not entirely destroyed), you can boot a dormant copy and "activate" it.
"CAUTION: FOLLOW INSTRUCTIONS CAREFULLY. IF YOU TRY TO FOOL THE SYSTEM YOU MAY BE LEFT WITHOUT AN 'ACTIVE' COPY."
"CAUTION: FOLLOW INSTRUCTIONS CAREFULLY. IF YOU TRY TO FOOL THE SYSTEM YOU MAY BE LEFT WITHOUT AN 'ACTIVE' COPY."
At this point, I would like to pause to point out that I am taking these screenshots in an emulator where I am booting a digital image of the original floppy disk which was made with a flux imager and what can only be described, in 1981 terms, as a supercomputer. So there's that.
Modern emulation is good enough that I can go through the entire process of making a dormant copy (to another .woz digital file). It boots to the same Protect-O-Disk screen, loads the entire game, then simply says "GOOD DORMANT COPY" and hangs.
You can boot the dormant copy to activate it. The first step, naturally, is to destroy your active copy.
The system is confused because my active copy has no read errors (which is true in the same way that eating a steak in The Matrix is delicious), but it'll let me activate a dormant copy anyway.
It has destroyed my active copy. Now the moment of truth...
It worked!
The former "active" copy is completely unbootable, and the former "dormant" copy is indistinguishable from the original (to the end user). Inspection with a nibble editor shows that the former "active" copy had an endless sequence of 0xFF bytes written to every track.
But wait, there's more! Protect-O-Disk is more deeply integrated into the game code than it appears. During boot, Protect-O-Disk checksums the peripheral slots on your Apple II. Later, in game, there are multiple calls to a similar routine. Any mismatch is fatal.
Protect-O-Disk also checksums your ROMs and compares them to a whitelist of acceptable ROMs. Which is why my original disk doesn't boot on my Apple //e – because this disk was released before my hardware and refuses to boot on unknown systems.
User-friendliness is relative.
User-friendliness is relative.
