Pool only shows one HDD, won't expand

[Chizard]

I have a 1TB pool. When I click show drives it only shows one drive in the pool:

ST1000DL002-9TT153 1TB

When I go to expand I try to use this drive under my unused drives: HGST HTS721010A9E630 1TB

It says the pool has expanded, the UI will update to show this but then,,, it just doesn't.

Any ideas what I need to do to have more than 1 drive in this pool?