# Fixing root filesystem errors with fsck on NixOS

If you run into an error like this:

```css
An error occurred in stage 1 of the boot process, which must mount the root filesystem on `/mnt-root` and then start stage 2. Press one of the following keys:

r) to reboot immediately
*) to ignore the error and continue
```

Then you can fix it like this:

1. Boot into a live CD/DVD for NixOS, or some other environment that has `fsck` installed, but *not* your installed copy of NixOS (as that will mount the root filesystem) [(source)](https://discourse.nixos.org/t/how-to-fix-a-slightly-broken-root-filesystem-on-nixos-vm-image/19972/4?u=joepie91)
2. Run `fsck -yf /dev/sda1` where you replace `/dev/sda1` with your root filesystem. [(source)](https://askubuntu.com/questions/885062/root-file-system-requires-manual-fsck)
    - If you're on a (KVM) VPS, it'll probably be `/dev/vda1`. If you're using LVM (even on a VPS), then you need to specify your logical volume instead (eg. `/dev/vg_main/lv_root`, but it depends on what you've named it).

<p class="callout warning">The above command will automatically agree to whatever suggestion fsck makes. **This can technically lead to data loss!**</p>

Many distributions will give you an option to drop down into a shell from the error directly, but NixOS does not do that. In theory you could add the `boot.shell_on_fail` flag to the boot options for your existing installation, but for reasons that I didn't bother debugging any further, the installed `fsck` was unable to fix the issues.