mdadm — Rebuilding a RAID Array After Reinstall¶

If you reinstall the OS on a machine that has an existing mdadm RAID array, the array metadata is still on the disks — you just need to reassemble it. The data isn't gone unless you've overwritten the member disks.

The Short Answer¶

# Scan for existing arrays
sudo mdadm --assemble --scan

# Check what was found
cat /proc/mdstat

If that works, your array is back. If not, you'll need to manually identify the member disks and reassemble.

Step-by-Step Recovery¶

1. Identify the RAID member disks¶

# Show mdadm superblock info on each disk/partition
sudo mdadm --examine /dev/sda1
sudo mdadm --examine /dev/sdb1

# Or scan all devices at once
sudo mdadm --examine --scan

Look for matching UUID fields — disks with the same array UUID belong to the same array.

2. Reassemble the array¶

# Assemble from specific devices
sudo mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1

# Or let mdadm figure it out from superblocks
sudo mdadm --assemble --scan

3. Verify the array state¶

cat /proc/mdstat
sudo mdadm --detail /dev/md0

You want to see State : active (or active, degraded if a disk is missing). If degraded, the array is still usable but should be rebuilt.

4. Update mdadm.conf so it persists across reboots¶

# Generate the config
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm.conf

# Fedora/RHEL — rebuild initramfs so the array is found at boot
sudo dracut --force

# Debian/Ubuntu — update initramfs
sudo update-initramfs -u

5. Mount the filesystem¶

# Check the filesystem
sudo fsck /dev/md0

# Mount
sudo mount /dev/md0 /mnt/raid

# Add to fstab for auto-mount
echo '/dev/md0 /mnt/raid ext4 defaults 0 2' | sudo tee -a /etc/fstab

Rebuilding a Degraded Array¶

If a disk failed or was replaced:

# Add the new disk to the existing array
sudo mdadm --manage /dev/md0 --add /dev/sdc1

# Watch the rebuild progress
watch cat /proc/mdstat

Rebuild time depends on array size and disk speed. The array is usable during rebuild but with degraded performance.

Gotchas & Notes¶

Don't --create when you mean --assemble. --create initializes a new array and will overwrite existing superblocks. --assemble brings an existing array back online.
Superblock versions matter. Modern mdadm uses 1.2 superblocks by default. If the array was created with an older version, specify --metadata=0.90 during assembly.
RAID is not a backup. mdadm protects against disk failure, not against accidental deletion, ransomware, or filesystem corruption. Pair it with rsync or Restic for actual backups.
Check SMART status on all member disks after a reinstall. If you're reassembling because a disk failed, make sure the remaining disks are healthy.

Reference: mdadm — How to rebuild RAID array after fresh install (Unix & Linux Stack Exchange)