i flashed a few of these with my on board sata (ich5). i had to go into my bios and mess with the sata settings in 'ide configuration'. your bios is most likely different from mine, but you should be able to find similar settings. i had to go from 'enhanced mode' to 'compatible' and set 'enhanced mode support' to 'pata only' from 'sata'.. i also unplugged all my hard drives and dvd drives leaving only the 360 and my windows hard drive.
while i was messing around with the settings i found there were times i couldnt get the slax disc to boot, or it would boot and go into mode b, but i couldnt see the drive in windows, or my bios would see the 360 drive but not my hard drive...once i found the way it would all work together it hasnt failed me since.