EMAS, and Multics. Two major operating systems that don't run anywhere, even under emulation.

We used to do stuff similar to this at Acorn in the 80's, both for multi-system and for the same system but with both 40 and 80 track drives. I vaguely remember that one key feature was that one system numbered tracks from 1 upwards and another from 0 upwards, allowing for two separate directories. There were a lot of tricks that I've forgotten now. You could sometimes fit more than 80 tracks on a floppy. And then there was the 9 vs 10 sectors per track issue. I was only peripherally involved in this area, but we had a couple of guys who knew it inside out. In fact our guys who wrote our disk protection mechanisms were damn good at breaking everyone else's too :-) I remember tricks with using different FDC controllers and even at one point sticking pins into floppies at exactly the right spot to force a hard error on a sector...