What I'm talking about has nothing to do with partitioning.
ADDING a formatted disk into a NEW slot in the parity array would invalidate parity. A clear disk has no impact on parity, so it is OK to add a clear disk, but a formatted disk is not clear.
So unRAID must clear the drive so parity remains valid.
The other possible method, which you could use if you wanted to ADD a disk with data on it, would be to New Config and rebuild parity.