root@xpc:~# zpool attach usb-zfs wwn-0x5000c500ab076db6 /dev/disk/by-id/wwn-0x50014ee658a36c74
root@xpc:~# zpool status usb-zfs
  pool: usb-zfs
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Sat Nov 17 22:48:13 2018
        67.9M scanned out of 265G at 579K/s, 133h19m to go
        67.6M resilvered, 0.02% done
config:

        NAME                        STATE     READ WRITE CKSUM
        usb-zfs                     ONLINE       0     0     0
          mirror-0                  ONLINE       0     0     0
            wwn-0x5000c500ab076db6  ONLINE       0     0     0
            wwn-0x50014ee658a36c74  ONLINE       0     0     0  (resilvering)

errors: No known data errors