string can phone

That klasikally-minded chap at OldSkool has, amidst a fascinating disquisition entitled Life Before Demos or Hobbyist Programming in the 1980’s, given a brief survey or variousfloppy-diskette copy-protection schemes:

• Simple trickery. Format a diskette as single-sided, but then store the secret copy-protection information on a track formatted on the second side. DISKCOPY would read the boot sector, determine that the disk was single-sided, and only bother copying the first side.
• User stupidity. The program would simply try writing a dummy file to the disk. Since most commercial software came on write-protected disks, the write would fail, and the program would continue. But if you’d just made a copy, chances were high that you’d forget to write-protect the disk when you were finished, and the write would succeed, which then aborted the program.
• Secret hardware information. Most disk drives could actually seek beyond track 40; usually to 41. Some software companies formatted that extra track (DISKCOPY didn’t go that high) and stored secret info in it for the program to check.
• Wacko disk formats. This is when you go slightly beyond the obvious, like formatting a normal 9-sector track with 10 or 11 sectors (or less, like 4 or 5), or by writing an incorrect track ID (track 20 says its track 30, etc.), or something similar. DISKCOPY didn’t know how to handle stuff outside the norm like this, so you usually needed a special program like CopyIIPC or CopyWrite to analyze the diskette thoroughly, then attempt to duplicate the format. This was about 66% successful; the other 33% you had to do yourself, usually (I personally used the Ultra Utilities). Electronic Arts had one of the
  best schemes in the early 80’s; I couldn’t figure it out until about 1985. They formatted track 15 with over 90 sectors! :-D
• Weak Bits. Some bits on the disk were recorded as halfway between 1 and 0, which made the disk difficult to copy. This wasn’t used too often because it was unreliable, even on the original disk!
• Damage. The most expensive method of copy protection was also the most effective: Physically damage the disk. Using a laser, it was possible to burn a small hole in the disk surface, and then all the program had to do was check to see if there was a read error in that particular sector, and if so, continue running the program. If you turned the disk surface manually by grabbing the inside ring, you could actually see a tiny hole in the disk surface!

We here at String Can Phone would never condone the breaking (or “cracking” in the vulgar parlance) of such schemes for purposes other than that of legally-protected backup, of course. Don’t Copy That Floppy!