ms28 help!

I've got an XBOX with a Toshiba-Samsung TS-h943a with the ms28 firmware...
I've followed the directions from Textbooks XBOX 360 firmware tutorial...
I have a VIA Sata VT6421 raid controller....
More to the point, I've created the bootable USB with iPrep... when i boot, if I just let the bios load it comes up with the message "No Boot Device Available. Press Enter to restart"
then it lists my SATA devices....

Then if I boot and press F12 for Boot Sequence, I can select the USB device and boot from that... However, when I do that I get an error saying "Missing Operating System"

I've tried downloading all the iPrep / firmware etc a 2nd time and I've looked through forums and google and can't find a remedy to this problem
please help!