A similar thing happend to me, too after I started using a WD MyBook Studio II (2TB) with Snow Leopard and then upgraded to Lion. I assumed the cause of the problem is the firmware. Have you applied the latest firmware? If you haven’t updated, do so. It may solve the problem. In my case, I wan’t able to apply it becuase it failed while applying. I contacted WD and exchaned a couple of emails but they couldn’t solve the problem.