BTX halted problem with 5.3-beta1

I get the infamous BTX halted problem when booting the 5.3b1 CD in a USB 
connected CDROM drive on the Supermicro P4SCi motherboard.
It works with a regular ATAPI CDROM. The older P4SGE boards works well 
with USB-CD booting (at least after i fixed cdboot a couple of months ago).

I found that turning off legacy USB helped someone in the past but it 
didn't do anything for this board. I also found that turning off ATA DMA 
mode might help but this BIOS don't let me do that.

Does anybody know what is causing this and a possible cure?
I'm going to try to debug this over the weekend so if I can get some 
clues to work with before friday I'm hoping to get a head start.

/Martin