When a device is replaced, a resilvering operation is initiated to move data from the good copies to the new device. This action is a form of disk scrubbing. Therefore, only one such action can occur at a given time in the pool. If a scrubbing operation is in progress, a resilvering operation suspends the current scrubbing and restarts it after the resilvering is completed.
For more information about resilvering, see Viewing Resilvering Status.