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I remember something about the MSX mania disks having an invalid media ID byte, indicating that the disk is double sided while in fact it is single sided. Early versions of openMSX tripped on this.

That's correct; the media id says double sided, while it's a 360k image. When emulators started emulating the disk controlers they tripped on that. The dsk2rom diskrom doesn't have problems with that though (tested), but maybe the .dsk implementation of the ocm trips on it

Never tried a .rom image on the ocm, maybe it has trouble booting up diskroms? A reset should solve that, but maybe it resets the header to prevent booting up roms after resetting the ocm?