first disk

root@hermes:/var/log # gpart destroy -F /dev/ada0
ada0 destroyed
root@hermes:/var/log # gpart create -s gpt ada0
ada0 created
root@hermes:/var/log # gpart show ada0
=>       40  250069600  ada0  GPT  (119G)
         40  250069600        - free -  (119G)

root@hermes:/var/log # gpart destroy -F /dev/ada0
ada0 destroyed
root@hermes:/var/log # gpart create -s gpt ada0
ada0 created
root@hermes:/var/log # gpart add -t freebsd-swap -s 2Gb ada0
ada0p1 added
root@hermes:/var/log # gpart add -t freebsd-zfs -l data ada0
ada0p2 added
root@hermes:/var/log # gpart show ada0
=>       40  250069600  ada0  GPT  (119G)
          40    4194304     1  freebsd-swap  (2.0G)
    4194344  245875296     2  freebsd-zfs  (117G)

root@hermes:/var/log # zpool create -f -m /mnt/data data /dev/gpt/data
root@hermes:/var/log # zfs list

NAME                 USED  AVAIL  REFER  MOUNTPOINT
data                 444K   113G    96K  /mnt/data
zroot               4.07G   252G    96K  /zroot
zroot/ROOT          3.23G   252G    96K  none
zroot/ROOT/default  3.23G   252G  3.23G  /
zroot/home           272K   252G    96K  /home
zroot/home/dschaft   176K   252G   176K  /home/dschaft
zroot/tmp            112K   252G   112K  /tmp
zroot/usr            858M   252G    96K  /usr
zroot/usr/ports       96K   252G    96K  /usr/ports
zroot/usr/src        858M   252G   858M  /usr/src
zroot/var           1.05M   252G    96K  /var
zroot/var/audit       96K   252G    96K  /var/audit
zroot/var/crash       96K   252G    96K  /var/crash
zroot/var/log        548K   252G   548K  /var/log
zroot/var/mail       140K   252G   140K  /var/mail
zroot/var/tmp         96K   252G    96K  /var/tmp

root@hermes:/var/log # ls -l /mnt/
total 4
drwxr-xr-x  4 root wheel 4 Jun 30 15:31 certs
drwxr-xr-x  2 root wheel 2 Jul  4 11:30 data
drwxr-xr-x  3 root wheel 3 Jun 30 09:41 home
drwxr-xr-x  2 root wheel 2 Jun 30 13:58 samba 

root@hermes:/var/log # zpool list -v
NAME         SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
data         117G   444K   117G        -         -     0%     0%  1.00x    ONLINE  -
  gpt/data   117G   444K   117G        -         -     0%  0.00%      -    ONLINE
zroot        264G  4.07G   260G        -         -     0%     1%  1.00x    ONLINE  -
  ada1p3     147G  1.69G   145G        -         -     0%  1.14%      -    ONLINE
  ada2p3     117G  2.38G   115G        -         -     0%  2.03%      -    ONLINE

second disk in Mirror-pair

root@hermes:/usr/local/etc # gpart destroy -F /dev/ada3
ada3 destroyed
root@hermes:/usr/local/etc #
root@hermes:/usr/local/etc # gpart create -s gpt ada3
ada3 created
root@hermes:/usr/local/etc # gpart add -t freebsd-swap -s 2Gb ada3
ada3p1 added
root@hermes:/usr/local/etc # gpart add -t freebsd-zfs -l data0 ada3

add it besides data disk in data pool

root@hermes:/usr/local/etc # zpool attach -f data /dev/gpt/data /dev/gpt/data0
root@hermes:/usr/local/etc # zpool status
  pool: data
 state: ONLINE
  scan: resilvered 984K in 00:00:00 with 0 errors on Fri Jul  4 13:22:04 2025
config:  

        NAME           STATE     READ WRITE CKSUM
        data           ONLINE       0     0     0
          mirror-0     ONLINE       0     0     0
            gpt/data   ONLINE       0     0     0
            gpt/data0  ONLINE       0     0     0 

errors: No known data errors 

  pool: zroot
 state: ONLINE
config: 

        NAME        STATE     READ WRITE CKSUM
        zroot       ONLINE       0     0     0
          ada1p3    ONLINE       0     0     0
          ada2p3    ONLINE       0     0     0 

errors: No known data errors
root@hermes:/usr/local/etc # ls -l /mnt/data/