Dual Disk Redundancy Greyed?

I’m trying to enable dual disk redundancy on my DroboPro but despite having 25% drive space free and an even combination of drives (4x 3tb, 4x 2tb) Dual Disk redundancy is still greyed out.
Is there a way of finding out how much space I need to enable this?

you will need to add a new HDD to turn on DDR

I’ve added 2x new 3TB drives to gain more free space in the last couple of days.
The enclosure already had 8 drives in it before this upgrade. It’s annoying Drobo Dashboard doesn’t say why its happening.