Solution is to force grub install on all drives.
After installimage, you need to change root directory to fresh installation, and run grub-install on all disks.
root@rescue ~ # installimage
# Assuming /SDC is SDD
# set->SWRAID 0, Comment DISK[n], ensue DISK1 is on SDD
root@rescue ~ # mount /dev/sdc3 /mnt/
root@rescue ~ # mount /dev/sdc2 /mnt/boot/
root@rescue ~ # cfdisk /dev/sda
# delete all. create single partition.primary. write. quit.
root@rescue ~ # cfdisk /dev/sdb
# delete all. create single partition.primary. write. quit.
root@rescue ~ # cd /mnt/
# set all proc/sys/dev avaliable from fresh install mount
root@rescue /mnt # mount -t proc /proc proc/
root@rescue /mnt # mount --rbind /sys sys/
root@rescue /mnt # mount --rbind /dev dev/
# https://www.ibm.com/docs/en/zos/2.3.0?topic=descriptions-chroot-change-root-directory-execution-command
root@rescue /mnt # chroot /mnt/
root@Ubuntu-1212 / # grub-install /dev/sdc
Installing for i386-pc platform.
Installation finished. No error reported.
root@Ubuntu-1212 / # grub-install /dev/sdb
Installing for i386-pc platform.
Installation finished. No error reported.
root@Ubuntu-1212 / # grub-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.
root@Ubuntu-1212 / # exit
root@rescue ~ # reboot
ps. I deliberately leaving comments and shell info, because you need to understand what each command do.
