RAID problems

I have raid 1+0 on an Asus P5W deluxe MOBO using the onboard chip, the problem is it drops one of the drives to non raid on every reboot, always the same drive, I have replaced the drive with a different one and it still has the same problem(abeit the drive was identical) The drive the drops is always known as external disc 0 whereas the others are know by the brand of drive (It should be the same as the top one as it is the same drive.)

The only thing I notice is that Native command queing support is disabled on both the western digital drives. Any ideas?

I'm running vista 64 ultimate.

Cheers
Chris