How to swap the "Unassigned" drive so that is is primary mirror for MBWEII (White)

I have a MBWEII (White Lights) - Containing 2 drives [A] & [B] configured as RAID1

I removed [A] and the made some changes to files on drive [B]

The I swapped over so that [A] was in the box and [B] removed and made some changes to files on drive [A]

When I replace [A] & [B] I get the following @

[A] HDD1 - Datevolume - Good

[B] HDD2 - Unassigned - Good … then the option to clean.

If I clean [B] then it deletes all the data and creates a mirror of [A] … so drive [A] is mirrored - cool !

But what if I want drive [B] to be the version to mirror rather than [A]

I can’t find a way to format / delete partitions for A & it is always the drive that the MBWEII wants to use as the primary disk to mirror from - how can I swap this.