6 disk array with 2 parity drives, one of the disks was showing "Metadata corruption detected" in the logs and showing "Unmountable: Unsupported partition layout" on the main array device screen.
I stopped the array and unassigned the 4TB disk from disk slot 3 and started the array so it was showing disk 3 as emulated.
While the disk was emulated I formatted the now unassigned disk that was from slot 3.
Stopped the array again and added the now formatted 4TB disk back to slot 3.
I started the array which initiated a data-rebuild, it's done about 854 GB (21.3 %) but I cannot see any of this being written onto disk 3.
Have I made a mistake and it's actually just removing the 4TB from parity, and not restoring the data from parity?