How do I rebuild a software RAID 1 array after a disk failure?

Issue

• One of the software RAID disks broke and now the system does not boot.

• mdraid was configured for /boot and root filesystem. The disk /dev/sda was recently removed to check if the system could boot using the remaining disk in mdraid, and the test was successful. But it is not possible to add the /dev/sda2 partition back into the mdraid array md0 and the error below was presented:

  $ mdadm --manage /dev/md0 --re-add /dev/sda2
  $ mdadm: --re-add for /dev/sda2 to /dev/md0 is not possible