SteamOS Dual Boot Installer Patch

Patch the official SteamOS recovery installer so you can install SteamOS into already‑created free space on your SSD — without wiping your existing Windows install.

✅ You must shrink your Windows C: partition yourself in Windows and leave Unallocated space before booting the SteamOS recovery image. This script does not resize partitions; it only patches the installer to target the free space you prepared.

Supported Devices

• ✅ Tested by me on ROG Ally X
• ⚠️ Expected to also work on other ASUS and Lenovo devices (similar setup, but not tested by myself personally)

Why this exists

Valve’s recovery image assumes it owns the whole disk and wipes everything. Reinstalling Windows afterwards is slow and unnecessary. This project patches the recovery’s repair_device.sh so it installs SteamOS into your unallocated free space instead of erasing the disk.

What the script does (high‑level)

• Detects the highest existing Windows partition number.
• Creates the standard SteamOS partition set after your existing partitions:
  • esp, efi-A, efi-B, rootfs-A, rootfs-B, var-A, var-B, and home (remaining space).
• Formats the new partitions and patches the recovery’s repair_device.sh to target them.
• Invokes the installer to lay down SteamOS into those partitions.
• Does not shrink/resize Windows or touch existing Windows partitions.

Defaults: TARGET_DISK=/dev/nvme0n1, sizes can be overridden via env vars: ESP_SIZE, EFI_SIZE, ROOT_SIZE, VAR_SIZE.

Prerequisites (do these in Windows before recovery)

1. Disable device encryption
   • Go to Settings > Privacy & Security > Device Encryption and turn it off (wait for full decryption).
2. Resize the Windows partition
   • Open Disk Management (Win+X > Disk Management or diskmgmt.msc).
   • Right‑click the C: partition > Shrink Volume… and choose how much free space to create (values are entered in MB).
   • Wait for the shrink to finish and confirm the new space shows as Unallocated (leave it unformatted).
     ⚠️ If C: is almost full the shrink can fail—free up space (uninstall games, etc.) before retrying.
3. Disable Fast Startup in Windows
   • Go to Control Panel > Power Options > Choose what the power buttons do.
   • Click “Change settings that are currently unavailable” (if shown) and uncheck “Turn on fast startup (recommended)”.
4. Disable Secure Boot in BIOS
   • Reboot, enter BIOS (hold your device’s BIOS key, e.g. VOL+ on Ally/Deck‑like devices), and disable Secure Boot.
5. Disable Fast Boot in BIOS
   • In the same BIOS session, locate the Fast Boot setting and turn it off. Save and exit.
6. Create a SteamOS recovery USB
   • Follow Valve’s instructions: SteamOS Recovery Instructions and write the image to a USB drive.

Install (from SteamOS Recovery)

1. Boot the device from the SteamOS recovery USB (Secure Boot must be off).

2. Open a terminal (Konsole) from the application menu.

3. Run:

   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:

   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
   

4. The script will show the highest existing partition and ask for the starting number for new SteamOS partitions (default is correct in most cases). Confirm to proceed.

5. When prompted, it will patch the installer and start the SteamOS install targeting the new partitions.

6. Reboot when done. Use your boot menu/manager to select SteamOS or Windows.

Demonstration Video

I have recorded a quick run through of this script on my ROG Ally X. You can watch it here:

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

Configuration (optional)

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

• Ensure the free space you created is truly unallocated and located after the Windows partitions.
• This is not an official Valve workflow. Use at your own risk.

Credits

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