Can I replace my drives?

I’m on TrueNAS-13.0-U6.2. I have (8) 2TB drives I’d like to replace with 4TB (or maybe bigger if budget allows). The pool has (4) mirrors, each with (2) drives in them.
I just want to make sure I"m doing this correctly before screwing something up. So I remove 1 drive & replace it with the new bigger drive, then hit the replace option in the UI. Wait a few hours for reselvering, and then repeat 7 more times. Then I’ll have double the capacity. Is that correct?

Correct, but if you have a spare SATA port the best way is to plug in the new drive without removing the old one. Then replace from the GUI; ZFS will automatically offline the old drive when resilver is done. That way there’s no loss of redundancy during the process.

Expansion is per vdev, so you only need two new drives to benefit from increased capacity.

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