Replacing/adding smaller boot drive

[Mcsneeze]

Hello,

Kind of a noob at this so be nice please

I have a boot zfs pool (rpool below) made up of two mirrored 1TB HDD's I salvaged out of old laptops. I would like to add a 3rd mirror to this pool that is a smaller SSD at 256GB as I don't trust the old HDD's.

I've read this forum post and have come to the conclusion that using the 'changing a failed bootable device' method might be the best, just with zpool attach instead of zpool replace. However, this step requires cloning the partition table of one of the healthy drives onto the new SSD.

Is this still possible given the size difference?
Do I need to rewrite the HDD partition tables to only use 256G?
Is it easier to just do a fresh install on the SSD, and then add a mirror (of the HDD's or another SSD) later? (if so, can someone point me in the direction of a guide to re-link the existing VM's/zfs pool on ZFS_TEST)


Any help is appreciated.

root@mcsneeze:~# zpool status
  pool: ZFS_Test
 state: ONLINE
  scan: scrub repaired 0B in 00:01:05 with 0 errors on Sun Jan 12 00:25:06 2025
config:

        NAME                                    STATE     READ WRITE CKSUM
        ZFS_Test                                ONLINE       0     0     0
          ata-Inland_SATA_SSD_IB24AK0004S00171  ONLINE       0     0     0

errors: No known data errors

  pool: rpool
 state: ONLINE
  scan: scrub repaired 0B in 00:02:24 with 0 errors on Sun Jan 12 00:26:27 2025
config:

        NAME                                               STATE     READ WRITE CKSUM
        rpool                                              ONLINE       0     0     0
          mirror-0                                         ONLINE       0     0     0
            ata-ST1000LM035-1RK172_WDEH9Z69-part3          ONLINE       0     0     0
            ata-HGST_HTS721010A9E630_JR1000BNGD3TAE-part3  ONLINE       0     0     0

errors: No known data errors

root@mcsneeze:~# zfs list
NAME                     USED  AVAIL  REFER  MOUNTPOINT
ZFS_Test                2.59T  1.02T    96K  /ZFS_Test
ZFS_Test/vm-101-disk-0  65.0G  1.06T  21.5G  -
ZFS_Test/vm-102-disk-0   520G  1.46T  67.8G  -
ZFS_Test/vm-103-disk-0  1.98T  2.99T  11.2G  -
ZFS_Test/vm-200-disk-0  32.5G  1.04T  5.08G  -
rpool                   7.58G   892G    96K  /rpool
rpool/ROOT              2.50G   892G    96K  /rpool/ROOT
rpool/ROOT/pve-1        2.50G   892G  2.50G  /
rpool/data                96K   892G    96K  /rpool/data
rpool/var-lib-vz        5.03G   892G  5.03G  /var/lib/vz

root@mcsneeze:~# zpool list
NAME       SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
ZFS_Test  3.72T   105G  3.62T        -         -     0%     2%  1.00x    ONLINE  -
rpool      928G  7.58G   920G