The IDE port has three address lines giving 8 16 bit registers directly addressable. Not all of them are decoded, there are some holes. I can't remember off the top of my head which ones, but I exploited that fact ten years ago to produce what was effectively an IDE-connected VGA card for the empeg, which could be connected to the IDE bus without interfering with the drives

Now, it may be that the problem is essentially that the converter is nobbling one or more address lines that don't affect the IDE port, being a non-decoded/non-used location, but the same lines mapped into the ethernet address space prevent it from working correctly. As I remember the ethernet controller uses a lot of registers, and it would only take one to screw it up completely.

Also, I very vaguely seem to recall that some of the IDE operations are 8 bit only, although I may be misremembering that. So again, if the higher data bits had an issue that might not affect the IDE to the point of killing it completely, although this seems much more unlikely to me.

pca