Storage > View VDEVs
Click the single Disk > Click Extend under ZFS Info > Select the New Disk > Extend
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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):
glabel status
3. GUI: Export Pool: fake
4. SSH: Attach fake to raid
zpool attach -f raid gptid/raid gptid/fake
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