Same vein for DOS: https://en.wikipedia.org/wiki/IBM_Extended_Density_Format https://en.wikipedia.org/wiki/IBM_Extended_Density_Format https://en.wikipedia.or

Same vein for DOS: https://en.wikipedia.org/wiki/IBM_Extended_Density_Format https://en.wikipedia.org/wiki/IBM_Extended_Density_Format https://en.wikipedia.org/wiki/2M_(DOS) https://en.wikipedia.org/wiki/2M_(DOS) https://retrocomputing.stackexchange.com/questions/12768/what-other-tricks-were-there-to-get-more-data-onto-floppy-disks https://retrocomputing.stackexchange.com/questions/12768/wha... I recall using an "extended" track to write hidden serial numbers to distribution disks. Not copy protection but phone support authentication.