1.replace the bad hd

2.sync partition table from the last available drive in the raid

#sfdisk -d /dev/sda | sed 's/sda/sdb/g' | sfdisk /dev/sdb

3.add new partition to the raid
#mdadm --manage /dev/md126 --add /dev/sdb1

4.install grub bootloader to the new drive
#grub2-install /dev/sdb