[On PET/CBM disks] "The ID contained in a track and sector is written once only – when the disk is NEW/HEADER-ed or created with a BACKUP/DUPLICATE. It can never be written again; disk writes don't touch this special area.
If you wanted to write a non-standard ID into a track-sector header, you'd have to do it by generating your own NEW formatting routine. This is not an easy trick, since it involves downloading a program into the inner processor of the disk. It would almost certainly involve destroying all information written on at least one track of the disk." Jim Butterfield