How to Fix 'Root Device Not Found' Error in Arch Linux

By Huzi

When Arch Linux cannot find the root device during boot, it's usually because the initramfs (the initial temporary file system) doesn't have the necessary components to locate or access your root partition. The table below outlines the most common causes and their solutions.

Cause of Issue	Primary Solution	Key Things to Check
Missing Hooks in mkinitcpio.conf	Add hooks (`lvm2`, `encrypt`), then regenerate initramfs.	Check for LVM, software RAID, or encrypted setups.
Incorrect `root=` Parameter	Fix the `root=` parameter in your bootloader config with the correct UUID.	Verify UUID with `blkid` and ensure it matches.
Hardware Detection Timing	Add kernel parameter `rootdelay` for slow-initializing drives.	Relevant for older hardware or USB drives.

Check and Fix Your Bootloader Configuration

The kernel parameter that specifies the root device might be incorrect.

Verify the Root Partition's UUID: From a live USB environment, use the blkid command to find the correct UUID of your root partition.
Check Bootloader Entry: Inspect your bootloader configuration file:
- For systemd-boot: Check the entry in /boot/loader/entries/arch.conf.
- For GRUB: Check /boot/grub/grub.cfg (usually generated from /etc/default/grub).

Ensure the root=UUID=... parameter matches the UUID from blkid exactly. Even a single wrong character will cause the error.

Regenerate Your Initramfs with Correct Hooks

The initramfs needs the right "hooks" to prepare your system's root device.

Identify Needed Hooks: From a live USB, chroot into your installed system and check /etc/mkinitcpio.conf. The HOOKS array must include hooks for your specific setup:
- LVM: If your root is on a logical volume, you must have the lvm2 hook.
- Encryption: For an encrypted root, you need the encrypt hook (or sd-encrypt).
- RAID: For software RAID, the mdadm_udev hook is required.
Add Hooks and Rebuild: Edit the HOOKS array in /etc/mkinitcpio.conf, placing necessary hooks (like lvm2) between block and filesystems. Save the file and regenerate the initramfs:
```
mkinitcpio -P
```

Troubleshoot Hardware and Timing Issues

If your storage hardware is slow to initialize, the kernel might not find the root device in time.

Use a rootdelay: Add rootdelay=10 (or a higher number of seconds) to your kernel parameters in the bootloader configuration. This tells the kernel to wait before looking for the root device.
Check Filesystem Hooks: Ensure the HOOKS array in mkinitcpio.conf contains fsck and filesystems.

Advanced Diagnostics

If you're still stuck, more advanced diagnostics can help pinpoint the problem.

Enable Debug Output: Add systemd.log_level=debug to your kernel parameters. This produces a very detailed log of the boot process.
Use a Recovery Shell: Kernel parameters like rescue or emergency can drop you into a shell during the initramfs stage, allowing you to manually check which devices and modules are available.

The key is to methodically check each part of the chain. By ensuring your mkinitcpio.conf hooks and bootloader root= parameter are correct, you can solve this common Arch Linux boot issue.