Yeah, GEMDOS already does this. It's one of the things I was planning to fix. It also hardcodes the hard disc to use FAT12 which is perhaps also not ideal.Please don't do what Lisa Office System and Xenix did - they hard coded the expected drive size inside the driver :facepalm: - as a result, if you attach a 30MB or 40MB SunRem MFM drive you could only use the first 10MB.
I was slightly afraid this was going to be the answer. Oh well, time to learn!