Extra Tracks

Tracks 1-35 were standard. Many software houses used 36-38 as well. Anything beyond 38 was risky as some drive heads could not reliably move beyond track 38. – Jim Drew

Bad Tracks where typically “Sync-Less” Tracks.

Non-Standard format

Later on, only track 18 (the directory track) was standard! Everyone used custom disk formats to get better speed and store more data. – Jim Drew

Half Track Protection

Half tracks are something that the 1541 could not easily write without modifying the drive, or being able to get lucky and come up with a routine that would the half tracks 180 degrees out of phase (like I did with the Bounty Bob Strikes Back! copier. -Jim Drew)

Density – Frequency Protections

Changing the density level was a common trick for putting more data on a track, as well as preventing copiers from clocking in the data properly. – Jim Drew

Clock Bits Protection

This is a protection where invalid GCR was used. Most commonly known as ‘weak bit’ protection. This was the most difficult protection to detect unless you were using a 1541-II or 1571 disk drive. Neither of these drives would ‘float’ invalid bits like a 1541 does. So, you can easily spot these and correct the data before being re-written. – Jim Drew