Instructions for Adding HDDs to RAID

[AdequateAndy]

I've got a F4-424 Pro NAS which was initially set up with an 8TB HD (Seagate 8TB 3.5" 7200rpm SATA IronWolf NAS HDD PN ST8000VN004).

I'm in the process of trying to add two additional HDs to my NAS (Seagate Ironwolf Pro Serial ATA III 22000 GB Internal Hard Drive, 3.5 Inch Size ).

Both of these are appearing in the Dick section in settings with Bay #2 being Good and Bay #3 being In risk.
When I try to add these to the existing storage pool, I get through to the confirmation page and I click confirm.

When I do this, the array synchronization never begins. What am I doing wrong?

Thanks for your help!