How to fix the "Windows could not prepare the computer to boot into the next phase of installation. To install windows, restart the installation" error.

Question

I currently have a 2019 Intel-Based iMac and have tried to run windows 10 on it without bootcamp assistant. When I try to use bootcamp, I get the error, "Partition failed, please try again." I have tried to use bootcamp multiple times, but it just won't work. So instead I am using a bootable windows 10 usb flash drive I also created on the iMac to install windows. In Disk Utility, I manually made my own 128gb partition to install windows on. When I restart my iMac, I hold down the option key and go into "EFI Boot" to launch the windows 10 setup when the usb drive is plugged in. I connect an external mouse so I can continue with the setup and select the partition I made earlier with Disk Utility so I can Dual-Boot MacOS and Windows 10, but once the installation almost reaches the end, I get the error "Windows could not prepare the computer to boot into the next phase of installation. To install windows, restart the installation"

Any help would be appreciated!

Answer 1

This is something which would be best addressed to the Apple Support folks, especially if you have some reason to avoid using Boot Camp Assistant. Procedure also may vary depending on specific macOS involved among other Mac-specific factors.

Answer 2

Hello, Agamjeet Mann

Thank you for your feedback. I understand that you want to inquire about dual booting on your iMac. Here are some troubleshooting solutions:

1. Run the “First Aid” feature in Disk Utility.

This is the most important step. A partition failure likely means that there are some minor errors in the disk directory structure.

Open “Applications” > ‘Utilities’ > “Disk Utility.”

In the left sidebar, click “Show” at the top and select “Show All Devices.”

From top to bottom, select your internal hard drive (e.g., APPLE SSD...), the container (Container disk...), and your Macintosh HD volume.

Click the “First Aid” button in the toolbar for each one and run it. Wait for it to complete the check and repair.

2. Clean up disk space and delete local snapshots

Boot Camp requires a large amount of contiguous available space. Even if the total available space is sufficient, if it is fragmented, the partition may fail.

Check space: Ensure you have at least 20-30GB more available space than the planned partition size.

Delete Time Machine local snapshots: This is the most common cause of partition failure. Even if you are not connected to a Time Machine hard drive, the system will create snapshots on the local disk.

Open Terminal (located in Applications > Utilities).

Enter the following command to view the snapshot list:

tmutil listlocalsnapshots /

If you see any snapshots (e.g., com.apple.TimeMachine.2025-07-20-xxxxxx), delete them one by one. Use the following command, replacing xxxxxx with the snapshot's date and timestamp:

sudo tmutil deletelocalsnapshots xxxxxx

(You will need to enter your Mac login password; characters will not be displayed as you type.)

After deleting all snapshots, try running Boot Camp Assistant again.

3. Ensure no other external drives are connected

When running Boot Camp Assistant, disconnect all unnecessary external drives, USB drives, and Thunderbolt devices, leaving only the power cable, keyboard, and mouse connected.

4. Try in Safe Mode

Restart your iMac, immediately hold down the Shift key after hearing the startup sound, and keep holding it until you see the Apple logo and progress bar. After entering Safe Mode, it will perform some self-checks and cache cleanup. Then restart normally and try Boot Camp Assistant again.

If Boot Camp still fails: Fix your manual installation method

If none of the above methods get Boot Camp Assistant working, you can continue to try a manual installation, but you'll need to adjust key steps.

The core issue is that you pre-formatted the partition in macOS Disk Utility. The Windows installer needs to handle the target space itself to create the multiple partitions it requires (a primary partition and a small system reserved partition).

 

Follow these revised steps:

Preparation:

Delete the 128GB partition you previously created using Disk Utility. Return to Disk Utility, select the partition, click the “-” button to remove it, and restore it as part of the macOS container. Your goal is for the Windows installer to see an “unallocated space” (Unallocated Space).

Ensure your Windows 10 installation USB drive includes Apple's Windows Support Software. You can run Boot Camp Assistant separately and select only “Download the latest Windows support software,” then copy the entire downloaded WindowsSupport folder to the root directory of your USB drive. This is crucial for installing drivers.

Begin installation:

Insert the USB drive, restart the iMac, and hold down the Option key.

Select the yellow “EFI Boot” icon to boot.

Critical partitioning steps:

In the Windows installer's “Where do you want to install Windows?” interface, you will see all the partitions on your Mac.

Locate the largest partition you intend to use for installing Windows (it should be the space previously allocated for Macintosh HD, now displayed as “Unallocated Space” or a partition you recognize).

Do not click Next directly! Select this space.

Click “Delete” at the bottom. If it is a partition, delete it to make it “Unallocated Space.”

Now, select this “Unallocated Space” and click “New.” The Windows installer will prompt you to create additional system partitions; click “OK.”

At this point, the installer will automatically create several partitions (a small system reserved partition and a large main partition). It will automatically select the large primary partition.

Ensure the primary partition is selected, then click “Format” to have the installer format it using the NTFS file system.

After formatting is complete, click “Next.”

Since the partition environment was created and prepared by the Windows installer itself, it can correctly write the boot files, and the installation process should complete smoothly.

After installation

After successful installation and booting into Windows, you may find that many devices are not functioning properly (e.g., Wi-Fi, sound card, poor graphics performance, etc.). At this point, open your USB drive, locate the WindowsSupport folder, and double-click the Setup.exe file inside to install all official Apple drivers.

I look forward to your feedback.

Best Regards, 

Ian | Microsoft Q&A Support Specialist