Reconfigure Windows to use AHCI¶

If the Ubuntu installer detects RST, and Windows is installed on your system, these steps enable Ubuntu to install side by side with Windows, without any loss of data and functionality.

To learn more about RST and its behavior with Ubuntu, see Intel RST during Ubuntu installation.

Back up your data¶

Any hard drive structure or configuration change, or installation of new operating systems on a hard drive that already contains data might potentially lead to a data loss.

Make sure that your personal data is safe. Even simply copying the important files to an external drive minimizes the risk of data loss.

Find the current controller mode¶

Open the Device Manager.
Under IDE ATA/ATAPI controllers, verify which controller mode is in use in Windows.

If the controller mode is set to anything other than Standard SATA AHCI Controller, you need to make a change that allows Windows to boot safely in AHCI mode. This can be done using the Registry Editor.

Change the controller mode¶

Start Registry Editor, and navigate to the following path:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\iaStorV\

Regedit

In the right column, double-click the Start key, and change its value to 0.
Double-click the iaStorV entry in the left column to expand it and select the StartOverride entry.
In the right column, change the value of the key 0 to 0.
Repeat this set of changes for the following path in the Registry Editor:
```
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\storahci\
```
Reboot Windows and start your computer’s firmware (BIOS or UEFI).

Usually, you can access the firmware by pressing the F2 or Del key during the early boot sequence.
In the firmware menu, change the disk controller type to AHCI.

The exact terminology and steps to manage the controller type in the firmware depend on the implementation by the platform vendor.
Exit the firmware and let the system boot.

Windows should start normally.
Check the controller mode in the Device Manager.

It should read: Standard SATA AHCI Controller.

Troubleshooting boot problems¶

After making the necessary changes to enable Ubuntu to install side by side with Windows, Windows might no longer boot. For instance, this might happen if you made the firmware change without making the registry changes in Windows. In this case, you need to recover your Windows installation.

The INACCESSIBLE BOOT DEVICE error screen

You will most likely see a blue screen with a Stop code: INACCESSIBLE BOOT DEVICE.

Windows attempts to restart and automatically diagnose and repair the boot-related problems, but it will probably fail to complete the task itself. You need to manually launch the command prompt from the recovery screen and fix the issue.

Open the command prompt¶

On the screen that gives you the result of the Automatic Repair, click Advanced options.
From the Choose an option screen, go to Troubleshoot ‣ Advanced options ‣ Command Prompt.

Diagnose the problem¶

The Windows command prompt opens. Here, you can run commands to diagnose and repair problems, including boot-related issues.

The command prompt

From the command prompt, run the disk partition tool to see and understand the disk layout:
```
diskpart
```
Display the list of volumes:
```
list volume
```
Make sure the volume that contains Windows is correctly assigned the letter C:.
If the Windows volume isn’t C:, you need to change this using the following commands:
```
select volume [number]
```
```
assign letter=[letter]
```
For example, a “wrong” volume might be assigned the letter C:. Select it first, assign it a different letter (e.g. F: or H:), select the volume that contains Windows, and then assign it the letter C:.
Activate the C: volume:
```
activate
```
Exit the diskpart utility:
```
exit
```

Repair the problem¶

Run the bcdedit repair command:

bcdedit /deletevalue {default} safeboot

If the previous command does not work, try alternative commands:

bcdedit /deletevalue {current} safeboot

bcdedit /deletevalue safeboot

If the previous command completes successfully, exit the command prompt, the Windows recovery console will restart, and Windows should load normally, with the controller mode set to AHCI.

If Windows does not start correctly, you can then manually recreate the bootloader file.

Recreating the bootloader file¶

To this end, you need to access either the System partition or the EFI partition on your computer. The Windows disk layout typically includes one of these two configurations:

A volume that contains Windows (C:) and a small hidden partition, usually 100MB in size called System partition, formatted as NTFS. It contains the files needed to start (boot) Windows, as well as recovery tools to help you diagnose and repair your system when it does not start correctly.
On UEFI-based computers, a volume that contains Windows (C:) and a boot partition (also called EFI), typically 256-512MB in size, formatted as FAT32. This partition contains the files needed to start (boot) Windows.

You need to access the partition to make the necessary changes.

If you have already run the diskpart utility, you do not need to do anything at this point, you only need to check the letter that is assigned to this partition (e.g. letter F:).

Assuming letter F: for the boot partition, use the following commands:

Switch to the boot partition:
```
F:
```
```
cd boot
```
Move the existing boot file aside:
```
ren BCD BCD.bak
```
Create a new one:
```
bcdboot c:\windows /l en-us /s f: /f ALL
```
The bcdboot command initializes the system partition by using BCD files from the C:\Windows folder.

The options used in the command:

/l en-us
Use the en-us locale.

/s f:
Target the system partition assigned letter F:.

/f ALL
Create boot files both for UEFI and BIOS.
Once the previous command completes, reboot. Windows should start normally.
You can now re-launch the Ubuntu installer and finish the side-by-side setup of the two operating systems.