You've got to have something holding up that PCI riser. With the fat back and a card installed in the standard single slot riser the fat back holds the riser in place. Running it open, with a slightly longer card (the two slot is too tall to fit in the fat back IIRC) you're putting all the stress on the slot itself, which isn't good, but what's worse is that if part of the back of your USB card happens to come in contact with the metal HD bracket you're risking a short.