Hi,
I am bumping this thread for advise because I have a very similar problem on a Duo-R. Most of the HuCard will load with no problem but one will always fails (Yokai Dochuki) and show a white screen sometimes with a backgroup sound (kind of beeeeeeeeeeeeeeeeeeeeeeeeeep).
I know the HuCard is not the problem because I have 2 copies of Yokai and they load perfectly on my CoreGrafx. I have tested about 10 HuCard and Yokai is the only one to exhibit this problem so far, but I did not have time to test more.
I tought it could be a bad solder on the HuCard connector so I used a rework station and the hot air gun on the solders. Strangely the HuCard will load just after but when everything cools down, I get again the white screen.
I have no idea were to look at. How is it possible that some components work when hot ?
thesteve: what (where) are "upper address line"? And which are the RAM chips on the motherboard?