Are you using an 80 wire cable?
40 wire cables are limited to ATA33 (UDMA 2)
And if you are, is it correctly laid out - BLUE to motherboard, as if not correctly installed, it may not be detected as 80 wire.
And try taking the slave drive off - I have a vague recollection that higher ULTRA modes (those needing an 80-wire cable) may be restricted if a non-ultra drive is on the same cable.