Dual boot with Windows

Windows and Arch on one disk, picked in the GRUB menu at power-on.

• Windows is already on the disk: install Arch next to it. Free space in Windows (2.3), then partition with option C.
• Arch is already installed: this page adds Windows 11 to it, with or without a USB stick.
• Only a few Windows apps? A virtual machine is simpler: VirtualBox comes with the dev group.

The base group has every tool used here:

Package	What it does here
os-prober	finds Windows so grub-mkconfig adds it to the menu
ntfs-3g	lets os-prober read the Windows (NTFS) partition
efibootmgr	lists the firmware boot entries and sets their order
dosfstools	mkfs.fat formats the FAT32 partition or stick UEFI boots the installer from
wimlib	wimlib-imagex split cuts the Windows image into parts that fit FAT32's 4 GB file limit
parted	moves partition boundaries and creates partitions (part of Arch's base install)
e2fsprogs	e2fsck checks and resize2fs resizes ext4 (part of Arch's base install)

Before you start

• Back up /home: resizing a partition can lose data.
• Pick a size for Windows: 64 GB is its minimum, 150–250 GB leaves room for apps and games.
• Windows 11 needs UEFI and TPM 2.0 (fTPM or PTT in the BIOS). Secure Boot can stay off.
• Download the ISO from microsoft.com/software-download/windows11.

Free space for Windows

The kit's layout puts /home last on the disk (EFI, /, /home), so Windows gets space cut from the end of /home. ext4 only shrinks while unmounted, and your session keeps /home busy: log out, switch to a console with Ctrl + Alt + F3 and log in as root. No root password yet? Set one first with sudo passwd root.

lsblk                                  # which partition is /home, here nvme0n1p3
parted /dev/nvme0n1 unit MiB print     # the "Disk /dev/nvme0n1:" line is the disk size

The new end of /home is the disk size minus the Windows size, minus 10240 MiB more if you have no USB stick (the installer partition goes there). For a 2 TB disk (1953514 MiB), 200 GiB for Windows (204800 MiB) and no stick: 1953514 − 204800 − 10240 = 1738474 MiB.

loginctl terminate-user <your-user>         # ends your processes, which keep files in /home open
umount /home                                # resize2fs can't shrink a mounted ext4
e2fsck -f /dev/nvme0n1p3                    # resize2fs refuses to shrink without a fresh check
resize2fs /dev/nvme0n1p3 1600G              # shrink the filesystem below the new partition size
parted /dev/nvme0n1 resizepart 3 1738474MiB # move the end of /home; answer Yes
resize2fs /dev/nvme0n1p3                    # grow the filesystem back to fill the partition
reboot

The filesystem shrinks first, to a size between what /home uses (df -h /home) and the new partition size, so cutting the partition can't cut off its end; the last resize2fs takes the margin back.

/ is the last partition (no separate /home)? It can't be unmounted from the running system: run the same steps from a live USB stick (the Arch ISO or GParted Live).

Installer media

USB stick

A stick of 16 GB or more: the Windows 11 ISO is about 9 GB. On a Windows PC, Rufus writes the ISO; Ventoy takes it as a copied file. From Linux, give the stick a FAT32 partition, the only filesystem UEFI is sure to boot from:

lsblk                                                   # find the stick, e.g. sda: NOT your system disk
sudo parted /dev/sdX mklabel gpt mkpart WIN11 fat32 1MiB 100%   # wipes the stick: new GPT, one partition
sudo mkfs.fat -F32 -n WIN11 /dev/sdX1                   # FAT32 with the label WIN11

Then copy the installer files to /dev/sdX1.

No stick: an installer partition

A 10 GiB FAT32 partition goes right after /home, in front of the Windows space; GRUB starts the installer from it. It holds the whole ISO (about 9 GB, check yours with ls -lh ~/Downloads/*.iso), and even one edition of Windows doesn't fit in 8 GiB. Creating it doesn't need the console, the disk can stay mounted:

sudo parted /dev/nvme0n1 mkpart WININSTALL fat32 1738474MiB 1748714MiB   # 10 GiB right after /home
sudo mkfs.fat -F32 -n WININSTALL /dev/nvme0n1p4                          # FAT32 with the label GRUB looks for

Add a GRUB entry for it: sudoedit /etc/grub.d/40_custom and append

menuentry "Windows 11 installer" {
    insmod fat
    insmod chainloader
    search --no-floppy --label WININSTALL --set=root
    chainloader /efi/boot/bootx64.efi
}

insmod loads GRUB's FAT reader and its chainloader, search finds the partition by its label, and chainloader hands the boot over to the Windows installer's EFI loader on it. Rebuild the menu (-o is where it's written):

sudo grub-mkconfig -o /boot/grub/grub.cfg

This is GRUB only: systemd-boot can't start a loader from another partition.

Copy the installer files

The Windows image, sources/install.wim, is bigger than 4 GB and doesn't fit FAT32 in one piece. Everything else is copied as is, and wimlib cuts the image into install.swm, install2.swm… of 3800 MB, which the installer reads as one:

sudo mkdir -p /mnt/iso /mnt/win
sudo mount -o loop,ro ~/Downloads/Win*.iso /mnt/iso         # the ISO as a read-only disk
sudo mount /dev/nvme0n1p4 /mnt/win                           # the installer partition, or the stick
sudo rsync -r --exclude=/sources/install.wim /mnt/iso/ /mnt/win/
sudo wimlib-imagex split /mnt/iso/sources/install.wim /mnt/win/sources/install.swm 3800
sudo umount /mnt/iso /mnt/win                                # flushes everything to the disk

The trailing / in /mnt/iso/ makes rsync copy the folder's contents, not the folder.

Install Windows

1. Boot the stick (F12, F8 or Esc at power-on) or pick Windows 11 installer in GRUB.
2. Choose the custom install, select Unallocated space, Next.
3. Leave every other partition alone (EFI, Linux, WININSTALL): no Delete, no Format.

Windows puts its boot files on the existing EFI partition and often makes itself the first boot entry. If the PC now starts Windows directly, fix the order below.

In Windows

In a terminal opened as administrator:

powercfg /h off
reg add "HKLM\System\CurrentControlSet\Control\TimeZoneInformation" /v RealTimeIsUniversal /d 1 /t REG_DWORD /f

• powercfg /h off turns off hibernation and fast startup with it. With fast startup Windows doesn't shut down but hibernates, and leaves its partition locked.
• RealTimeIsUniversal makes Windows keep the hardware clock in UTC, as Linux does. Without it the clock jumps by your time zone every time you switch systems. /v is the value name, /d 1 its data, /t REG_DWORD a number, /f skips the confirmation.
• Device encryption or BitLocker on (Settings → Privacy & security → Device encryption)? Save the recovery key from account.microsoft.com/devices/recoverykey: started from GRUB, Windows may ask for it once.

Back to GRUB

Press F12, F8 or Esc at power-on and pick GRUB (the entry may be called GRUB or arch). In Arch:

sudo efibootmgr                    # entries (Boot0000 GRUB, Boot0001 Windows Boot Manager…) and BootOrder
sudo efibootmgr -o 0000,0001       # -o sets the order: your GRUB number first, Windows Boot Manager next
sudoedit /etc/grub.d/40_custom     # remove the "Windows 11 installer" entry, if you added it
./install.sh --only boot           # rebuilds the GRUB menu; look for "Found Windows Boot Manager"

./install.sh --only boot runs grub-mkconfig with os-prober turned on; sudo grub-mkconfig -o /boot/grub/grub.cfg does the same. A big Windows update can put Windows Boot Manager first again: the same efibootmgr -o fixes it.

Give the installer partition back to /home

Once Windows works, the 10 GiB go back to /home. ext4 grows while mounted, so this runs from your normal session:

lsblk                                                # the WININSTALL partition, here nvme0n1p4
sudo parted /dev/nvme0n1 rm 4                        # delete it
sudo parted /dev/nvme0n1 unit MiB print free         # the "Free Space" line right after /home: note its End
sudo parted /dev/nvme0n1 resizepart 3 1748714MiB     # /home ends where that free space ends; answer Yes
sudo resize2fs /dev/nvme0n1p3                        # grow the filesystem into it

parted refuses an end that runs into the next partition, so a wrong number fails instead of touching Windows.

Secure Boot

The kit runs with Secure Boot off, and Windows 11 installs that way. Some games' anti-cheat (Valorant, Battlefield 6, recent Call of Duty) won't start without it. Turning it on means signing GRUB and the kernel with your own keys (sbctl), which the kit doesn't do.