Hi, if it is still relevant:
Well, I had a similar problem.
1) The drive should not be formatted as MBR. http://forums.debian.net/viewtopic.php?f=30&t=103875
2) when I was copying the .iso image on the disk formatted with mbr I copyed it into /dev/sdX1, which is wrong, it should be copied to /dev/sdX
I lost quite some time with that.... hope it helps.