Adding a disk to an existing zpool

zpool attach <zpool name> <existing device> <new device>

Where <existing device> is gpt/disk0 in the zpool status output below.

 pool: zroot
 state: ONLINE
 scrub: none requested
config:

    NAME         STATE     READ WRITE CKSUM
    zroot        ONLINE       0     0     0
      gpt/disk0  ONLINE       0     0     0

The pool will create a 1-way mirror (essentially RAID1) using the now two devices and immediately resilver the data onto from the existing disk onto the new.

 pool: zroot
 state: ONLINE
 scrub: resilver completed after 0h0m with 0 errors on Fri Jan 26 15:43:54 2013
config:

    NAME           STATE     READ WRITE CKSUM
    zroot          ONLINE       0     0     0
      mirror       ONLINE       0     0     0
        gpt/disk0  ONLINE       0     0     0
        gpt/disk1  ONLINE       0     0     0  592K resilvered