Hey,
I have an SD HxC as well which I sometimes use as DF1 & 2 externally, but with a custom cable. I see no reason why it wouldn't work with an external drive's board.
Are you sure that:
- You have the HxC jumpers set so it responds to SEL0?
- You are using the cable before the twist?
- The cable is inserted the right way around? (I wouldn't just trust the notches and red stripe alone here)
- You're HxC's config is correct?
- The drive's board and the HxC are in working order.