zpool-attach —
    attach new device to existing ZFS vdev
  
    zpool | 
    attach [-fsw]
      [-o
      property=value]
      pool device new_device | 
  
Attaches new_device to the existing
    device. The existing device cannot be part of a raidz
    configuration. If device is not currently part of a
    mirrored configuration, device automatically
    transforms into a two-way mirror of device and
    new_device. If device is part of
    a two-way mirror, attaching new_device creates a
    three-way mirror, and so on. In either case,
    new_device begins to resilver immediately and any
    running scrub is cancelled.
  -f 
  - Forces use of new_device, even if it appears to be
      in use. Not all devices can be overridden in this manner.
 
  -o
    property=value 
  - Sets the given pool properties. See the
      zpoolprops(7)
      manual page for a list of valid properties that can be set. The only
      property supported at the moment is
      ashift.
 
  -s 
  - The new_device is reconstructed sequentially to
      restore redundancy as quickly as possible. Checksums are not verified
      during sequential reconstruction so a scrub is started when the resilver
      completes. Sequential reconstruction is not supported for raidz
      configurations.
 
  -w 
  - Waits until new_device has finished resilvering
      before returning.