well there is a jumper settin to limit ur hdd size to a particular limit depending on the hdd
if u dun wanna use all of ur hdd
the JUMPERS are meagrely shorting 2 pins ... what happens is inside the disk
the internal cicuitry restricts access to the remainin 8 gb in this case when the particular pins are shorted
anyways its not ur mobo problem ... im quite convinced of that. ..
i can only see a jumper problem

if i think of anything else ill post. ..
do let us knwo if anything works