My Bad. I meant system bios, as this is a chipset sata raid, not a dedicated controller card. I've added the fourth drive now, and set the system bios to manage the drive. HDtune.com benchmarks show the drive is performing up to speed.
My thinking, which may be wrong, is that there are two settings for the drive in the system bios. system controlled, or raid controlled, (or disable). I'm not sure what would happen if I handed the disk over to the "raid controlled" setting. Would existing data survive? Would I still have a second drive and drive letter? Would I/O operations be more efficient than "system controlled" under heavy loads?
the best thing i guess would be to backup everything and start hacking. this is an nVidia nForce 590 chipset on the motherboard. Performs quite well with sata300 750B barracudas X 3 raid5. HDtune reports around 140M/s and i think around 12ns.
My assumption is that having the 590 chipset manage all the drives would be optimal, but there is little documentation that i can find on the matter. thanks for the reply NST.