## Summary

Omarchy 4.0.1 installer's partition resize operation corrupts BTRFS filesystems by changing the partition table without properly moving/resizing the filesystem data. This causes existing CachyOS installations to fail with "failed to mount /sysroot" on next boot.

---

## Hardware

- **Machine:** Ryzen 11 desktop
- **Disk 1 (nvme0n1):** 1TB — Windows 11 (NTFS partitions) + leftover ZFS label
- **Disk 2 (nvme1n1):** 4TB — CachyOS root on p5 (BTRFS, 681GB) + NTFS data

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## What happened

### Attempt 1: Free-space install on nvme1n1

1. Booted from Omarchy USB installer
2. Used `Ctrl+Alt+F2` → `cfdisk` to shrink existing partition and create free space
3. Selected "Free space install" option in installer
4. **Bug:** Installer modified partition table (changed partition size) without properly shrinking the BTRFS filesystem first
5. Install failed with mount errors
6. On reboot, CachyOS failed with: **"failed to mount /sysroot"**
7. Windows also failed to boot

### Root cause

The installer used `cfdisk` to change the partition boundaries, but the BTRFS filesystem inside the partition still expected data at the old disk locations. The partition size changed, but the filesystem metadata pointed to wrong disk addresses.

This caused:
- BTRFS superblocks appeared valid (`btrfs rescue super-recover` said "all supers are valid")
- But mounting failed with "wrong fs type, bad option, bad superblock"
- `btrfs check` reported "input/output error, cannot open filesystem"

### Recovery

Used `testdisk` to restore the original partition boundaries. This aligned the partition table back with where the BTRFS data actually was on disk. CachyOS booted successfully after partition table restoration.

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## Error from installer log

```
archinstall.lib.exceptions.DiskError: Could not mount /dev/nvme1n1p5: mount /dev/nvme1n1p5 /mnt/arch_btrfs
['/usr/bin/mount', '/dev/nvme1n1p5', '/mnt/arch_btrfs'] exited with abnormal exit code [32]: mount: /mnt/arch_btrfs: wrong fs type, bad option, bad superblock on /dev/nvme1n1p5
```

**Log:** https://logs.omarchy.org/dwCUFn

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## The correct resize procedure (for reference)

For BTRFS, resize must happen in this order:

1. **Shrink filesystem FIRST:**
   ```bash
   sudo btrfs filesystem resize 300G /
   ```

2. **Then shrink the partition to match:**
   ```bash
   sudo parted /dev/nvme1n1 resizepart 5 300GB
   ```

The Omarchy installer appears to do these in the wrong order, or skips the filesystem resize entirely.

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## Impact

- Corrupts existing BTRFS installations when attempting partition resize
- Breaks boot of existing OS (CachyOS, Windows)
- Requires `testdisk` or similar recovery tool to restore partition boundaries
- Potential data loss if user doesn't know how to recover

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## Workaround

Before running the Omarchy installer on a disk with existing BTRFS partitions:

1. Boot from live USB
2. Manually shrink BTRFS filesystem first: `btrfs filesystem resize <size> /mnt`
3. Then shrink the partition with `parted` or `gdisk`
4. Run Omarchy installer on the free space

Or use a dedicated disk with no existing partitions.

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Related: #8358 (installer confused by existing partition layout)