I set up mdadm on my Debian but made an error that I cannot get corrected.
Indeed the root partition is on a one-way mirror /dev/md2 because when I created the system I only had a single harddisk. I now know that I should have created the mirror with a "missing" component but it is too late now, and as it is my root partition I am somewhat reluctant to twiddle it very much; nor is it easy to experiment because is and must be in use.
I can add the second part:
mdadm --add /dev/md2 /dev/sdb1
but that will add the second component as a spare, not as an active element. I tried lots of tweaking /etc/mdadm.conf but to no avail.
What is the way to add an active component to an mdadm level 1 array?