Thank you for your replies and I apologize for my tardiness in responding
On the advice of the installer, due to the "swappable" nature of the disk array, I have set up my devices under "Removable Backup to disk" devices.
I cannot see a way to create the "backup to Disk" folders, while using a "removeable backup to disk" devices - it says I can only use the "root" drive when I try to do so.
Am I doing something wrong ?
Thanks again