The manual has no screen shots on adding a vdev to an existing pool.
I did a quick search and didn't find any obvious screenshots from other posters, though I have read about the new GUI not being as clear as is should be for this process.
I don't have any free ports or drives so I can't simulate and provide screenshots.
The key outcome is to add a (2) drive mirror, not a single drive. You check the outcome with "zpool status" at the console.
Should look something like this:
Code:
Pool Name
mirror vdev
drive 1
drive 2
mirror vdev
drive 1
drive 2
Here is a video that seems to cover some of the basics, I didn't watch/listen to the video (can't stand youtubers) just skipped through without sound. He is adding (3) drive RaidZ1 vdevs rather than mirrors, it's the same idea. This is last years 11.1 "new" GUI not the current 11.2 "new" GUI but they seem mostly the same.
https://www.youtube.com/watch?v=76qnBjZF65g