i think it's a bug in the older motherboard bios actually.
i've used 33 and 66 drives together, and even ata66 and cd-roms together.
but I've seen probs, at times, with the seagate 10gb drives (off ebay) (ex-xbox?) these drives behave weirdly.
possibly short changed internally by seagate as they were 'supposed' to be for a specific use and not generally for sale.
so the problem shouldn't have arrived.
but modders are a strange lot.