Software Raid1 & booting 2nd mirror2nd disk

I know Hardware Raid is but but for some application Software Raid is useful
and generally works ok

I manually created a Broken Raid1 mirror using a LiveCD
single partition raid md1

Installed OpenSuse 15.2 by selecting keep existing partitions

Resulting installtion boots fine.
I have added the 2nd disk using mdadm /dev/md1 --add /dev/sdb1

check with cat /proc/mdstat - miiror is complete

I tried Yast boot loader & selecting sdb but the disk does not boot!!
first disk sda does but not sdb

any ideas would be welcome