Convert single drive stripe to mirror
Storage > View VDEVs
Click the single Disk > Click Extend under ZFS Info > Select the New Disk > Extend
Convert single drive stripe to mirror when:
One disk exists in a single drive Pool named: raid.
One unused drive available of at least the same size.
1. GUI: Create a new Pool: fake
2. SSH: Get gptid of both (raid and fake):
3. GUI: Export Pool: fake
4. SSH: Attach fake to raid