Replacing a degraded drive in a software RAID 1 array

After physically replacing the degraded drive with the new one and rebooting the system, run the folowing command to create exactly the same partitioning on /dev/sdb (new drive) as on /dev/sda (healthy existing drive):

sfdisk -d /dev/sda | sfdisk /dev/sdb

You can run

fdisk -l

to check if both hard drives have the same partitioning now.

Next we add /dev/sdb1 to /dev/md126 and /dev/sdb2 to /dev/md127:

mdadm --manage /dev/md126 --add /dev/sdb1
mdadm --manage /dev/md127 --add /dev/sdb2


Now both arrays (/dev/md126 and /dev/md127) will be synchronized. Run

cat /proc/mdstat

to see when it's finished.