How to fix a raid1 mirrored boot disk with a failed single disk.
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Getting Info
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mdadm --detail /dev/md0
cat /proc/mdstat
fdisk -l
lsblk
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Recovery Steps
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Assume the mirrored disks are /dev/sda and /dev/sdb and
/dev/sdb disk failed
# may not be needed as the failed disk is already marked failed and removed
mdadm --manage /dev/md0 --fail /dev/sdb2
mdadm --manage /dev/md0 --remove /dev/sdb2
# physically replace /dev/sdb then mirror the partition table
shutdown the system and replace the failed disk, then do
sfdisk -d /dev/sda | sfdisk /dev/sdb
# check
fdisk -l
# add new disk to mirror disk md0
mdadm --manage /dev/md0 --add /dev/sdb2
# check and track progress of sync
cat /proc/mdstat
# After the raid completes, make /dev/sdb bootable also.
# If this is not a boot disk, then skip.
grub-install /dev/sdb
update-grub /dev/sdb