The trick is that the 765 FDC can read the custom byte values in the gaps, but it cannot write them. This made it hard for people to make working copies of protected disks.
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=== Weak Sectors Protection ===
Another popular copy-protection scheme was using (fully or partially) unmagnetized sectors. It involves leaving some portions of the track unmagnetized. The unmagnetized data would then appear as random values when read by the FDC.
When the protected program is launched, it reads these weak sectors multiple times. Due to their unstable nature, these sectors return different data on each read. If the program detects this changing data, it recognizes the disk as an original and continues to run. If not, the program can then crash, reset the computer, or do other protective measures.
The trick is that the FDC cannot unmagnetize portions of a track or leave portions of a track unmagnetized to recreate this effect on another floppy disk.
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