Jay, Dont you think todays hard drives still uses the MFM coding??
RLL (run length limit) was also MFM coding but they shrinked the code space on media by 50%, with MFM they used 5MHz crystal and in RLL they used 7.5MHz. Remaining all were same. I used a RLL controller card with a 20MB MFM hard drive and got 30MB space. It was wayback in 1990.

Rauf