(broken) PCs and writing DD 5.25" media

So, I've got a PC here with a brain-dead BIOS that only acknowledges 3.5" HD drives. Ugh. I can hook up a 5.25" HD drive instead and override the BIOS type when booting Linux (passing "floppy=0,2,cmos" to the kernel), and that seems to work. However, when hooking up a (good) 5.25" DD drive (and using "floppy=0,1,cmos") I consistently get I/O errors. Anyone have any theories as to what the problem might be? Maybe the FDC in the machine just plain doesn't support 250k/s transfer rates (unfortunately I don't have any 3.5" DD media to test that by formatting a disk in a 3.5" HD drive) - but that would seem like a weird feature to abandon? Formatting 5.25" DD media in the HD drive *appears* to work, but I assume that will be using a 300k/s rate rather than 250 (and of course the narrower head width makes it pointless anyway) I can haul another PC out of storage with a better BIOS/FDC, but I'm still curious as to what the problem might be with this setup. Linux just identifies the FDC as a "post-1991 82077" (for which 250k support is claimed), but I expect it's a reimplementation within a larger I/O chip rather than a discrete FDC IC. cheers Jules