Preparation
Before removing drive, remove from raid arrays:
mdadm -f /dev/mdX /dev/sdXX
mdadm -r /dev/mdX /dev/sdXX
Setup New Drive
Copy partitions from spare working to new drive:
sfdisk -d /dev/sdOLD | sfdisk /dev/sdNEW
Re-add drives to arrays:
mdadm -a /dev/mdX /dev/sdXX