OK, if the disk failed mid rebuild you can use the invalid slot command:   -Tools -> New Config -> Retain current configuration: All -> Apply -Assign any missing disk(s) -Important - leave the browser on that page -Open a SSH session and type: mdcmd set invalidslot 5 29 -Do not  check the "parity is already valid" and start the array, disk5 will rebuild and parity2 re-sync
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