A: When a disk fails from an aggregate, a hot spare is automatically pulled into the aggregate and depending on if it failed or went to pre-fail, will determine if a raid reconstruct (disk failed) starts or a disk copy (pre-fail).
Q: Spare disk not zeroed mean?
A: When a spare disk is "not zeroed" it means that the drive has not had blocks zeroed out. You do not need to zero out disks manually. ONTAP will automatically zero the disks before they are used within an aggregate.