and now we wait

zpool status

root@mfsbsd:~ # zpool status
  pool: system
 state: DEGRADED
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 Mon Feb 22 19:14:30 2016
        39.6G scanned out of 20.0T at 72.3M/s, 80h14m to go
        3.73G resilvered, 0.19% done
config:

        NAME                                              STATE     READ WRITE CKSUM
        system                                            DEGRADED     0     0     0
          raidz2-0                                        DEGRADED     0     0     0
            gptid/1f65b3e4-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/21f2788d-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/2484e4ef-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            replacing-3                                   OFFLINE      0     0     0
              1601580527160658707                         OFFLINE      0     0     0  was /dev/da3p3
              gptid/b0737f6b-d996-11e5-9b1d-00259082215a  ONLINE       0     0     0  (resilvering)
            gptid/29b08a29-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/2c5b024e-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/cf46c81a-d6ba-11e5-8298-00259082215a    ONLINE       0     0     0
            gptid/31e93100-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/329ad7d7-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/3412b7e6-efc8-11e2-92b7-00259082215a    ONLINE       0     0   112  (resilvering)

errors: No known data errors
root@mfsbsd:~ #

zpool status 2

root@mfsbsd:~ # zpool status
  pool: system
 state: DEGRADED
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 Mon Feb 22 19:14:30 2016
        689G scanned out of 20.0T at 298M/s, 18h50m to go
        65.2G resilvered, 3.37% done
config:

        NAME                                              STATE     READ WRITE CKSUM
        system                                            DEGRADED     0     0     0
          raidz2-0                                        DEGRADED     0     0     0
            gptid/1f65b3e4-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/21f2788d-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/2484e4ef-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            replacing-3                                   OFFLINE      0     0     0
              1601580527160658707                         OFFLINE      0     0     0  was /dev/da3p3
              gptid/b0737f6b-d996-11e5-9b1d-00259082215a  ONLINE       0     0     0  (resilvering)
            gptid/29b08a29-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/2c5b024e-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/cf46c81a-d6ba-11e5-8298-00259082215a    ONLINE       0     0     0
            gptid/31e93100-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/329ad7d7-efc8-11e2-92b7-00259082215a    ONLINE       0     0     0
            gptid/3412b7e6-efc8-11e2-92b7-00259082215a    ONLINE       0     0   112  (resilvering)

errors: No known data errors
root@mfsbsd:~ #

zpool status 3

  scan: resilver in progress since Mon Feb 22 19:14:30 2016
        1.13T scanned out of 20.0T at 319M/s, 17h11m to go
        110G resilvered, 5.65% done

root@mfsbsd:~ # zpool import -R /mnt system
root@mfsbsd:~ # mkdir /mnt/dev
root@mfsbsd:~ # mount -t zfs system/rootfs /mnt
root@mfsbsd:~ # ls /mnt/dev/
root@mfsbsd:~ # mount -t devfs devfs /mnt/dev
root@mfsbsd:~ # ls /mnt/dev/zfs
/mnt/dev/zfs
root@mfsbsd:~ # cp -R /mnt/boot /mnt/boot.orig
root@mfsbsd:~ # rm -rf /mnt/boot
root@mfsbsd:~ # mv /mnt/boot.orig /mnt/boot
root@mfsbsd:~ # ls /mnt/boot
beastie.4th     brand-fbsd.4th      device.hints~       loader          logo-beastiebw.4th  menusets.4th        version.4th
boot            brand.4th       dtb         loader.4th      logo-fbsdbw.4th     modules         zfs
boot0           cdboot          firmware        loader.conf     logo-orb.4th        pmbr            zfsboot
boot0sio        check-password.4th  frames.4th      loader.conf~        logo-orbbw.4th      pxeboot         zfsloader
boot1           color.4th       gptboot         loader.efi      mbr         screen.4th
boot1.efi       defaults        gptzfsboot      loader.help     menu-commands.4th   shortcuts.4th
boot1.efifat        delay.4th       kernel          loader.rc       menu.4th        support.4th
boot2           device.hints        kernel.old      logo-beastie.4th    menu.rc         userboot.so
root@mfsbsd:~ #