single drive dual boot for Omarchy + Windows (Intel Mac specifics)

[multinomi]

After many unsuccessful attempts to install Omarchy side-by-side with Windows, I tried to reverse the problem and install Windows after Omarchy. I tried this #900 partition resizing approach using Gparted Live but it did not work with Omarchy 3.1.7 on MacBookPro11,4 (15-inch, Mid 2015), here are my findings and solutions.

Problem

Using gparted to resize the Omarchy partition makes the GPT disk partition scheme "Hybrid MBR"

gdisk /dev/sda

Found valid GPT with hybrid MBR; using GPT

and then Windows installer sees this GPT as MBR and refuses to install Windows because it requires GPT partition scheme on UEFI systems.

Formatting the partition to NTFS does not help, the initial clean "protective MBR" became "hybrid MBR" every time I used gparted, even without creating any partitions afterwards. This is likely due to some backwards compatibility behavior of libparted, perhaps a hybrid MBR is automatically generated when modifying GPT disks, but I haven't explored further...

Solution

[optional] To speed up multiple reboots, I used an external SSD with Ventoy with Omarchy, Arch and Windows ISO images. On this MacBook, Ventoy boots these Linux ISOs only in GRUB2 mode, and Windows ISO only in WIM mode.

To resize the partitions I used sgdisk. (parted didn't work with partition resizing math, or perhaps I made a mistake.)

The manual resize is not trivial, since we have to get BTRFS < LUKS < GPT with the math in sectors, not MiB or GiB.

1. Install Omarchy automatically the normal way. The process takes <4 minutes on this 2015 MacBook Pro with a USB ethernet dongle.

2. Boot into Omarchy just to see if it works. No sense updating or setting anything up until the partition is successfully resized.

3. Reboot into Arch live ISO.

4. Unlock your encrypted root:

cryptsetup luksOpen /dev/sda2 cryptroot
mount /dev/mapper/cryptroot /mnt

5. Shrink Btrfs filesystem to 72 GiB (my example).

btrfs filesystem resize 72g /mnt
umount /mnt

6. Now shrink the Luks encryption container to be slightly larger than the filesystem container (72 GiB). Size is calculated in sectors, 512 bytes per sector:

72 GiB => 77,309,411,328 bytes ÷ 512 bytes/sector = 150,994,944 sectors + 244,736 sectors (≈ 120 MB container buffer) = 151,239,680 sectors.

cryptsetup resize --size 151239680 cryptroot
cryptsetup status cryptroot
cryptsetup luksClose cryptroot

(I noticed that rebooting into Omarchy before the next step (GPT partition resize) resets the LUKS container size to fill the partition again.)

7. Resize the 2nd partition with sgdisk: it will write whatever sector addresses you type, there are no safety measures!

Find the start sector of the Omarchy partition:

sgdisk -p /dev/sda

Note for partition 2:

Start sector: 4196352
Type code: 8304

Give the GPT partition an extra 212 MB (≈ 435 000 sectors) beyond the current container size.

SIZE = 151239680 # current cryptroot sectors
BUFFER = 435000 # ≈212 MB
END = START + SIZE + BUFFER - 1
END = 4196352 + 151239680 +435000 - 1 = 155871031

Note: the buffer size in step 6 and 7 is arbitrary 100-200 MB, but do check the numbers manually if you use AI for help. ;)

1 GiB (gibibyte) = 1024^3 = 1073741824 bytes = 2097152 sectors.

sgdisk commands

• Save the current PARTUUID:

OLDUUID=$(blkid -s PARTUUID -o value /dev/sda2)

• Rewrite partition 2 with that slightly larger boundary:

sgdisk -d 2 /dev/sda
sgdisk -n 2:4196352:155871031 -t 2:8304 -u 2:$OLDUUID /dev/sda
sgdisk -p /dev/sda

• Reload partition table and verify:

partprobe /dev/sda
cryptsetup luksOpen /dev/sda2 test
cryptsetup status test
cryptsetup luksClose test

You should see the LUKS device open normally (still ≈72 GiB).
This extra 212 MB margin makes the layout tolerant to any automatic rounding or header variations.

Result

Btrfs filesystem: 72 GiB (shrunk safely, inside LUKS).
LUKS container: ≈ 72.1 GiB (resized and verified).
GPT partition: ends slightly after the LUKS container (~+212 MB margin).
PARTUUID: preserved, so /etc/crypttab and initramfs still match.
Free space: everything after sector 155,871,031 is unallocated, ready for Windows.

8. Now reboot to Omarchy, check the partition size with lsblk or gdisk (install gptfdisk first).

Installing Windows

Windows installs with no issues onto the empty unpartitioned space. There is no need for a 2nd EFI partition, as it uses the existing EFI partition as intended.

Intel Mac needs rEFInd bootloader for VT-x

On this MacBook I have to use rEFInd as the primary bootloader to enable hardware virtualization support.

It is straightforward to add rEFInd to EFI partition and chainload Limine. I used Hasleo EasyUEFI for Windows to re-insert my previous rEFInd directory and make rEFInd the default bootloader. Here's my refind.conf for text-only boot menu and hiding several Limine duplicate entries:

timeout 5
use_nvram false

hideui all
textonly
textmode 0

enable_and_lock_vmx true

scan_all_linux_kernels false
scanfor manual

menuentry "Omarchy" {
    loader \EFI\limine\limine_x64.efi
    volume "EFI"
    icon \EFI\refind\icons\os_arch.png
    textonly
}

menuentry "Windows" {
    loader \EFI\Microsoft\Boot\bootmgfw.efi
    volume "EFI"
    icon \EFI\refind\icons\os_win.png
    textonly
}

Since Omarchy and Limine are left as installed, this setup is more likely to live through Omarchy updates.

Disclaimer

I have reconstructed these instructions from notes, the logic is solid, but some minor steps could be missed, as I didn't run it again after getting the result. Use AI to help you through this if you are unfamiliar with CLI disk tools.

BTW, this partition resizing process can be easily automated. Hint, hint...

[TheAhumMaitra]

You can look at this beautiful guide by me - Here

[multinomi]

I couldn't get 3.1.6 to install correctly from basic Arch installation, there were some script errors which interrupted the installer, and I tried correcting them manually, but then the final system didn't work properly. Not a beginner friendly approach. :)

Hence i prefer resizing the official install, it's quite easy, btw. 🔢

[TheAhumMaitra]

What errors you got?

[multinomi]

seems like the install script wasn't correctly handling installation options for 2 packages... i fixed those two packages by preinstalling them, but there were more install errors towards the end, but system booted but gave invalid config errors on startup...

like i said, if it worked out of the box with automatic ISO install, i wasn't going to spend hours trying to do manual installation, there are enough problems with configuring Apple hardware. 👎

[multinomi]

resizing the partition post automatic install which "just works" is like re-arranging luggage in the trunk of a car.
installing and configuring Arch manually is like assembling a car...