SteamOS Dual Boot Installer

Install SteamOS into already-created unallocated space on your SSD without wiping your existing Windows install.

Latest GUI build:

• Download the latest release

[!IMPORTANT] You must shrink your Windows partition yourself in Windows before booting the SteamOS recovery image. This tool does not resize partitions. It creates the SteamOS partition set inside prepared unallocated space and performs the install there.

Supported Devices

• Tested on ROG Ally X
• Tested on a DIY Steam Machine:
• AMD 5800X
• AMD RX 9060 XT
• 16 GB RAM
• 1 NVMe SSD shared by Windows, Bazzite, and SteamOS
• 2 additional SSDs for game storage
• Expected to also work on similar ASUS and Lenovo handhelds, but not personally tested on all of them

Why this exists

Valve’s recovery image assumes it owns the whole disk. That is fine for a clean install, but wasteful if Windows is already set up and you only want to install SteamOS into free space you prepared ahead of time.

This project provides two ways to do that:

• A GUI wizard that helps you pick the correct disk and unallocated region from inside SteamOS recovery
• A direct run.sh backend flow if you want to execute the installer manually

What the tool does

• Detects existing disks, partitions, and unallocated regions
• Creates the standard SteamOS partition layout after your existing partitions:
• esp, efi-A, efi-B, rootfs-A, rootfs-B, var-A, var-B, and home
• Formats the new partitions
• Copies the SteamOS recovery root filesystem into the new SteamOS partitions
• Finalizes the SteamOS boot configuration
• Does not shrink Windows or resize existing partitions for you

Default backend values:

• TARGET_DISK=/dev/nvme0n1
• ESP_SIZE=256M
• EFI_SIZE=64M
• ROOT_SIZE=11G
• VAR_SIZE=1G

Prerequisites

Do these in Windows before booting SteamOS recovery:

1. Disable device encryption. Go to Settings > Privacy & Security > Device Encryption and turn it off. Wait for decryption to fully finish.
2. Shrink the Windows partition. Open Disk Management, shrink C:, and leave the new space as unallocated. Do not create a filesystem there.
3. Disable Fast Startup in Windows. Go to Control Panel > Power Options > Choose what the power buttons do, unlock the protected settings, and uncheck Turn on fast startup.
4. Disable Secure Boot in BIOS.
5. Disable Fast Boot in BIOS.
6. Create a SteamOS recovery USB. Use Valve’s instructions: SteamOS Recovery Instructions
7. Configure Windows to use UTC for the hardware clock. Windows and Linux treat the RTC differently by default. If the clock is wrong after booting Linux, SteamOS recovery can fail HTTPS/TLS validation and report a connection problem even though Wi-Fi itself is working.

Reference: - Arch Wiki: UTC in Microsoft Windows

Administrator Command Prompt command:

bat reg add "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation" /v RealTimeIsUniversal /d 1 /t REG_DWORD /f

[!IMPORTANT] After applying the Windows UTC change, make sure the actual system time is correct before starting recovery. It is worth forcing Windows to resync the clock before rebooting. Change the time manually, then set it back correctly, or otherwise trigger a time resync, and verify the BIOS/system clock is correct before booting SteamOS recovery.

GUI Install Flow

This is the recommended way to use the project.

1. Boot from the SteamOS recovery USB.
2. Download the latest steamos-installer-wizard build from the repository releases page.
3. In the file manager, right-click the downloaded file, open Properties, go to Permissions tab, and allow it to be executed as a program.
4. Double click the steamos-installer-wizard file to run the wizard.
5. Let the wizard guide you through selecting the target disk and unallocated region, reviewing the install plan, and launching the installer.
6. Reboot when finished and select SteamOS or Windows from your boot menu.

Script Install Flow

Use this if you do not want to use the GUI, or if you want to simply run the installer script on its own (not really recomended tho).

1. Boot the device from the SteamOS recovery USB (Secure Boot must be off).
2. Open a terminal (Konsole) from the application menu.
3. Clone the repo and run the backend:

bash git clone https://github.com/Josh5/steamos_dual_boot_installer_patch cd steamos_dual_boot_installer_patch sudo ./run.sh

or if you want to run it in a single line:

bash wget -O /tmp/run.sh https://raw.githubusercontent.com/Josh5/steamos_dual_boot_installer_patch/refs/heads/master/run.sh && sudo bash /tmp/run.sh

The script flow assumes you already know which disk and free-space region you want to use. It also will assume you are targeting the first disk.

Videos

DIY Steam Machine Dual Boot

https://youtu.be/8_6u0za39JA

Dual Boot Ally X

https://www.youtube.com/watch?v=sVW2MKR5cNk

Unofficial Guides

https://www.youtube.com/watch?v=pd76H_FATT4

Script Configuration

Environment variables you can pass to run.sh:

• TARGET_DISK (default /dev/nvme0n1)
• ESP_SIZE (default 256M)
• EFI_SIZE (default 64M)
• ROOT_SIZE (default 11G)
• VAR_SIZE (default 1G)

Example:

sudo TARGET_DISK=/dev/nvme1n1 ROOT_SIZE=15G ./run.sh

Safety Notes

• Double-check that the space you prepared is truly unallocated before running the installer.
• Double-check that you selected the correct physical disk.
• Double-check the first new partition number shown in the review step or backend output.

[!WARNING] This is not an official Valve workflow. Use it at your own risk.

Credits

Created by Josh5 to save everyone from wasting a day reinstalling Windows just to try SteamOS next to it.