Few more great writeups:

https://map.grauw.nl/articles/low-level-disk/

https://www.unige.ch/medecine/nouspikel/ti99/disks.htm

https://scarybeastsecurity.blogspot.com/2021/05/recovering-l...

and my own contribution https://github.com/raszpl/sigrok-disk decoder for viewing floppy/hdd tracks in nice GUI using $5 logic analyzers like https://sigrok.org/wiki/Lcsoft_Mini_Board or https://github.com/gusmanb/logicanalyzer