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Mdadm Helpers

How do I replace a disk marked as removed from a linux md raid-5 array

First identify removed drive:

lsblk

Find serial number, if necessary:

smartctl -a /dev/sdb | grep Serial

If drive is just remved:

mdadm --zero-superblock /dev/sdXn
mdadm /dev/md0 --add /dev/sdXn

If replacing damaged drive, then only:

mdadm /dev/md0 --add /dev/sdXn

mdadm --examine /dev/sdX