http://www.xlr8yourmac.com/tips/Disable_WDGreen_HeadParking....
The default head parking is 10s. In a ZFS configuration, the other drives time out while waiting for the drive to wake up from a parked configuration. This causes a cascade of IO waves back and forth as the drives in a large array park and then unpark to respond, causing IO to plumment.
Works fine with 1 drive, or even 2 drives. Past that, the probability of a collision and retry on head parking escalates exponentially.