May I comment on these changes?

On the one hand, you are right that "20, READ ERROR", "21, READ ERROR" do not help the user a lot. OTOH, these are the official names, and whenever someone familiar with CBM drives sees them, they know what they are.

On the other hand, "20, NO SECTOR HEADER" is completely unknown, and might be confusing.

I ask myself if something like "20, READ ERROR (NO SECTOR HEADER)" and so on would not be better than replacing the error codes completely?

Just an idea,
- Spiro