Consider hddpool with special device as mirrors on 2 SSD partitions:
NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
hddpool 1.83T 294G 1.55T - - 22% 15% 1.00x DEGRADED -
mirror-0 1.81T 291G 1.53T - - 22% 15.7% - ONLINE
ata-TOSHIBA_HDWD120_81842ZHGS 1.82T - - - - - - - ONLINE
ata-ST2000DM008-2FR102_WFL5BSR0 1.82T - - - - - - - ONLINE
special - - - - - - - - -
mirror-2 20G 2.94G 17.1G - - 30% 14.7% - DEGRADED
ata-SSDPR-CX400-128-G2_GY2015459-part8 20.1G - - - - - - - ONLINE
ata-SPCC_Solid_State_Disk_30041226481-part8 20.1G - - - - - - - FAULTED
to provide redundancy add any storage to special device mirror-2 to turn it into 3-way mirror with one faulty device:
zfs create -V 21g nvmpool/special zpool attach hddpool ata-SPCC_Solid_State_Disk_30041226481-part8 /dev/zvol/nvmpool/special
Now it can wait for new disc.