---
title: "mdadm — Rebuilding a RAID Array After Reinstall"
domain: linux
category: storage
tags: [mdadm, raid, linux, storage, recovery, homelab]
status: published
created: 2026-04-02
updated: 2026-04-02
---

# 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

```bash
# 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

```bash
# 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

```bash
# 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

```bash
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

```bash
# 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

```bash
# 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:

```bash
# 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)](https://unix.stackexchange.com/questions/593836/mdadm-how-to-rebuild-raid-array-after-fresh-install)

## See Also

- [snapraid-mergerfs-setup](snapraid-mergerfs-setup.md)
- [rsync-backup-patterns](../../02-selfhosting/storage-backup/rsync-backup-patterns.md)