You're not trying to boot from the card are you? Because the 6421 chipset doesn't support that.
Also when i boot with the 360 disk drive already attached, my computer does this as well. So don't hook it up till after it gets past the cards bootloader.
If you have a pci sound card and video off of pci, it could be a IRQ problem and you may need to disable sound and if possible go to onboard video.
a few thoughts.