Back then, grub and initrd didn't understand mdadm so would be set to boot off one of the boot vols as a raw disk. Once initrd handed over to the root vol, it w

Back then, grub and initrd didn't understand mdadm so would be set to boot off one of the boot vols as a raw disk. Once initrd handed over to the root vol, it would understand mdadm properly. If a disk with a boot vol failed, it would have a 50/50 chance of still booting depending on which one failed. You'd create a backup grub entry to boot off the other one manually. Not as transparent as hardware RAID, and maybe now grub is aware enough to auto handle mdadm? I don't know - I haven't done bare metal mdadm for a decade or so.