Dual booting with Windows

Understanding Dual Booting

Dual booting means having two operating systems installed on the same computer and choosing which one to start each time you power on. In this chapter, the focus is on running Linux alongside an existing Windows installation on the same machine, without removing Windows.

At a high level, you will:

1. Prepare Windows so it can safely share the disk.
2. Shrink the Windows partition to free space for Linux.
3. Install Linux into the free space.
4. Configure (or accept automatic configuration of) the bootloader so you can pick Windows or Linux at startup.

This chapter assumes you already know how to create installation media and perform a basic Linux installation from the parent “Installing Linux” chapter.

Key Concepts for Dual Booting

EFI/UEFI vs Legacy BIOS (Why It Matters)

Modern systems typically use UEFI with a special EFI System Partition (ESP) that contains bootloader files for all installed operating systems.

For dual booting with Windows:

• Most Windows 10/11 systems are installed in UEFI mode.
• You must install Linux in the same mode (UEFI, not Legacy/CSM) so both systems can coexist cleanly.
• There is usually already an EFI System Partition created by Windows; Linux will reuse it.

You don’t need to fully understand UEFI internals, but you must:

• Avoid deleting or reformatting the EFI System Partition.
• Ensure you boot the Linux installer in UEFI mode if Windows is UEFI.

Preparing Windows for Dual Boot

Before making any changes, back up your important data. Partitioning and OS installation are generally safe but mistakes can be destructive.

Check How Windows Is Installed (UEFI or Legacy)

In Windows 10/11:

1. Press Win + R, type msinfo32, press Enter.
2. In System Information, look for “BIOS Mode”:
• UEFI → Windows uses UEFI.
• Legacy or CSM → Windows uses legacy BIOS mode.

Also check the partition style:

1. Right-click Start → Disk Management.
2. Right-click your disk (e.g., “Disk 0”) → Properties → Volumes.
3. Look at “Partition style”:
• GPT → typically used with UEFI.
• MBR → typically used with Legacy BIOS.

Linux must be installed in the same mode (UEFI with GPT or Legacy with MBR) for a smooth dual boot.

Disable Fast Startup

Windows Fast Startup can interfere with accessing NTFS partitions from Linux and can cause bootloader issues.

1. Open Control Panel → Hardware and Sound → Power Options.
2. Click Choose what the power buttons do.
3. Click Change settings that are currently unavailable.
4. Under “Shutdown settings”, uncheck Turn on fast startup (recommended).
5. Save changes.

(Optional but Recommended) Turn Off Secure Boot Initially

Many modern Linux distributions can work with Secure Boot, but disabling it simplifies troubleshooting, especially for beginners.

To disable Secure Boot:

1. Reboot and enter firmware settings (often by pressing F2, Del, Esc, or F10 during power-on; the key depends on your hardware).
2. Find Secure Boot in the Security or Boot tab.
3. Set it to Disabled.
4. Save and exit.

You can try re-enabling Secure Boot after installation if your distribution supports it.

Freeing Space for Linux from Windows

You usually do not want to delete your existing Windows partition. Instead, you shrink it to make unallocated space where Linux can be installed.

Steps in Windows:

1. Right-click Start → Disk Management.
2. Locate the main Windows partition:
• Usually labeled C:, type Basic, filesystem NTFS.
• Do not modify small system partitions (like “System Reserved” or 100–500 MB “EFI System Partition”).
3. Right-click the C: partition → Shrink Volume.
4. Enter the amount to shrink, in MB:
• For a basic desktop Linux install, allocate at least 20 GB (≈ 20,000 MB), preferably 40 GB+ if you can.
• Example: type 40000 for roughly 40 GB.
5. Click Shrink.

After shrinking:

• You should see Unallocated space on the disk.
• Leave this unallocated; do not create a new partition in Windows for Linux. The Linux installer will handle it.

Turn Off BitLocker (If Enabled)

If BitLocker is enabled on your system (common on some editions of Windows):

1. Open Control Panel → System and Security → BitLocker Drive Encryption.
2. If your system drive (usually C:) shows BitLocker as “On”, click Turn off BitLocker and decrypt.
3. Wait until decryption finishes before you resize partitions or install Linux.

Full disk encryption can complicate bootloader installation. If BitLocker must remain on, dual boot is still possible, but be prepared for more complex troubleshooting.

Installing Linux Alongside Windows

The exact installer screens differ between distributions, but the process and decisions are similar.

Boot the Linux Installer in the Correct Mode

From the parent chapter you know how to boot from the Linux USB. Here, the extra consideration is:

• If Windows is UEFI:
• Ensure the boot device entry says something like UEFI: <USB Name>.
• Avoid legacy/CSM entries for the USB.
• If Windows is Legacy BIOS:
• Boot the installer in legacy mode as well.

Mixing modes (Windows UEFI + Linux Legacy, or vice versa) prevents clean dual boot and usually leads to confusing boot behavior.

Choosing the Installation Type

Most beginner-friendly distributions (e.g., Ubuntu, Fedora, openSUSE) offer clear options such as:

• “Install alongside Windows” / “Install Linux alongside Windows Boot Manager”.
• “Erase disk and install Linux”.
• “Something else” / “Manual partitioning”.

Guidelines:

• Choose “Install alongside Windows” if:
• The installer correctly detects Windows.
• It shows a reasonable plan (e.g., using the unallocated space or shrinking partitions you expect).
• Choose “Manual partitioning” (or equivalent) if:
• You want to explicitly pick the unallocated space.
• The automatic option looks wrong or offers to remove Windows.
• Windows is not detected due to unusual partition layouts.

Never choose an option that says it will erase the entire disk unless you intentionally want to remove Windows.

Partitioning for Dual Boot

Detailed partitioning concepts are covered in the parent “Partitioning” and its child chapters. Here, focus on the dual-boot-specific decisions.

Reusing the EFI System Partition (UEFI Systems)

If your system uses UEFI, Windows has already created an EFI System Partition (ESP), typically:

• Size: 100–500 MB
• Type: EFI System Partition
• Filesystem: FAT32
• Contains: EFI/Microsoft/Boot/ and similar.

In the Linux installer’s partitioning step:

• Do not delete or format the EFI System Partition.
• Mark this partition as:
• EFI System Partition or Use as: EFI System Partition.
• Mount point often set to /boot/efi (if the installer asks).
• Linux will add its own bootloader files (e.g., EFI/ubuntu, EFI/fedora) into the existing ESP.

Creating Linux Partitions in the Unallocated Space

Use the unallocated space you created in Windows:

• At minimum, create:
• A root partition: mounted as /.
• Optionally a separate swap partition (or rely on a swap file, depending on the distribution defaults).
• Optionally a separate /home partition if you prefer.

Keep these points in mind:

• Do not modify or format the Windows C: partition (NTFS).
• Do not modify the EFI System Partition except to mount it for bootloader installation.
• Make sure the new Linux partitions are created only in the freed/unallocated space.

Common choices:

• Filesystem for root: usually ext4.
• Mount point: /.
• Size: whatever free space you decided to allocate.

Bootloader Considerations (GRUB and Others)

The bootloader is the program that appears when your computer starts, letting you choose between Linux and Windows.

Most distributions will:

• Install GRUB2 (or another bootloader) as the default.
• Detect Windows automatically and add a menu entry for “Windows Boot Manager” or similar.

Where the Bootloader Is Installed

On UEFI systems:

• GRUB’s EFI binary (e.g., grubx64.efi) is added to the existing EFI System Partition.
• An NVRAM boot entry like “ubuntu” or “fedora” is created.
• The firmware is usually configured to boot Linux first; GRUB then lists Windows.

On Legacy BIOS systems:

• GRUB is typically installed to the disk’s Master Boot Record (MBR).
• GRUB menu appears first; it includes Linux and Windows options.

When the installer asks:

• For UEFI: ensure the EFI System Partition is selected as the place for the bootloader files, not your Windows C: drive as NTFS.
• For Legacy: select the disk itself (e.g., /dev/sda) rather than an individual partition (like /dev/sda1), unless you have a specific advanced setup.

After Installation: Choosing Between Windows and Linux

After rebooting:

• Your system should show the GRUB menu (or your distribution’s bootloader menu).
• Entries might include:
• Your Linux distribution (default).
• “Windows Boot Manager (on /dev/… )” or similar.

You can:

• Use arrow keys to choose Linux or Windows.
• Press Enter to boot the selected one.
• Wait for the timeout to boot the default OS automatically.

If Windows does not appear:

• Boot into Linux.
• Update the bootloader configuration:
• On many distributions using GRUB:
• sudo update-grub (Debian/Ubuntu family).
• Or sudo grub2-mkconfig -o /boot/grub2/grub.cfg (Fedora, some others).
• Confirm that os-prober (a tool for detecting other OSes) is installed and enabled; some distributions disable it by default for security reasons.

Windows-Specific Issues That Affect Dual Boot

Windows Fast Startup and Hibernation

If Windows is in a hibernated or “Fast Startup” state, Linux may see NTFS partitions as “dirty” and mount them read-only, or refuse to mount them.

To avoid filesystem corruption:

• Always shut down Windows fully before booting Linux:
• Disable Fast Startup as shown earlier.
• Avoid using hibernate if you plan to access Windows partitions from Linux.
• If you see warnings in Linux like “The NTFS partition is in an unsafe state”:
• Boot back into Windows.
• Shut down using:
• Shift + Click on “Shut down” from the Start menu (forces full shutdown).
• Then try again in Linux.

Time Differences Between Windows and Linux

By default:

• Windows stores the hardware clock as local time.
• Most Linux distributions assume the hardware clock is UTC and apply timezone offsets.

This can cause the clock to appear wrong when switching between OSes.

Options:

1. Configure Linux to use local time for the hardware clock:
• Example (on many systems):
• sudo timedatectl set-local-rtc 1
2. Or configure Windows to use UTC (requires registry tweaks and is better for advanced users).

Troubleshooting Common Dual-Boot Problems

System Boots Directly into Windows (GRUB Not Showing)

Possible causes:

• Firmware (BIOS/UEFI) boot order changed to boot Windows first.
• Secure Boot or firmware update reset settings.
• Bootloader not installed correctly.

Steps to fix:

1. Enter firmware setup:
• Check the boot order.
• Move your Linux entry (e.g., “ubuntu”, “fedora”) above “Windows Boot Manager”.
2. If the Linux entry is missing:
• Boot the system from your Linux live USB.
• Use a boot-repair tool (some distributions have a package literally called boot-repair).
• Or reinstall the bootloader manually from a chroot (advanced).

Windows Missing from GRUB Menu

If Linux boots but you don’t see Windows in the GRUB menu:

1. Boot into Linux.
2. Install os-prober if your distribution uses it and it isn’t installed:
• Example (Debian/Ubuntu): sudo apt install os-prober.
3. Enable its use in GRUB if the distribution disables it by default (check /etc/default/grub for an GRUB_DISABLE_OS_PROBER line).
4. Regenerate GRUB configuration:
• sudo update-grub (Debian/Ubuntu).
5. Reboot and check the menu again.

Restoring the Windows Bootloader (If You Want to Remove Linux)

If you later decide to remove Linux and boot only Windows:

1. Boot into Windows installation or recovery media.
2. Use “Repair your computer” → “Troubleshoot” → “Advanced options” → “Command Prompt”.
3. Run:
• On UEFI systems:

     bootrec /fixboot
     bootrec /scanos
     bootrec /rebuildbcd

• On some systems, you may also need bcdboot C:\Windows.

4. Once Windows boots directly, you can delete Linux partitions using Windows Disk Management and extend the Windows partition into the freed space.

(Exact commands can vary across Windows versions; always verify against current Microsoft documentation.)

Best Practices and Safety Tips

• Back up important data before resizing partitions or installing another OS.
• Keep a bootable Windows recovery drive and your Linux live USB handy for repairs.
• Avoid touching:
• The EFI System Partition (other than marking it as ESP/mounting in the installer).
• The Windows recovery partitions.
• When in doubt at the partitioning step:
• Cancel the installation.
• Boot back into your existing OS.
• Verify your partition layout and free space.
• Document changes:
• Write down the original partition table (sizes, order) before installing.
• This can be invaluable for recovery if something goes wrong.

Dual booting with Windows is a powerful way to adopt Linux without giving up your existing environment. With careful preparation and attention to installation options, you can switch between both systems reliably on the same machine.