https://www.reddit.com/r/zfs/comments/1nkty6f/vestigial_pool_with_real_pools_device_as_a_member/
I've solved this and thought I'd leave my working here in case it's of use to anyone. It was with the help of ChatGPT which helped me with a crucial realisation. (That in itself was a learning experience - it appeared to show real understanding of zfs.)
As it turned out both disks had stale zfs labels on them from previous experiments I'd done, using the whole disk as the device. The reason they were left around is that the active pool is on the the first (and only) partition, which is aligned differently.
- The partition starts at sector 2048 (1MB into the device) - as is standard.
- It ends at the 12TB boundary, ~130MB before the end of these particular devices (WD Red). I sized it that way so that other 12TB disks would be attachable to the vdev in the future even if their exact size varied slightly.
Here's the partition layout: