How can I force a new installation of BIOS Grub etc on sda when all exist on sdb

I am trying to do a new installation of 13.2 on a rebuilt drive array, seen as sda in partitioner. My problem is I have a copy of the original 13.2 installation on second array seen as sdb in partitioner.
Even using expert mode the system seems reluctant to do what I want with warnings on subvolumes appearing even though I have edited all the sdb volumes so that they are not mounted. It also seems impossible to creat the BIOS Grub partition at the start of sda.
Any ideas how I should go about this please?