How to Fix Upgrade Error 0xC1900101–0x40021 on Windows 11

If you are trying to update Windows 11 but continually get the 0xC1900101–0x40021 error, where Windows 11 fails during the SECOND_BOOT phase of installation, specifically during the SYSPREP_RESPECIALIZE operation. This guide will show you quite a few different things you can do to solve this problem. Most of the time, the issue is a trivial one, so there's no need to panic too much just yet.

How to Fix Upgrade Error 0xC1900101–0x40021 on Windows 11

Once again, Microsoft has managed to make updating/upgrading Windows 11 more complicated than it really needs to be. This is something that seems to happen semi-regularly, so don't be too worried if you get this error message or a similar one. Simply work your way through the list of steps below and you should be able to fix the problem in a few minutes.

Update BIOS or UEFI Firmware

The first thing you need to do is to check for any BIOS or UEFI updates. If there are new updates, download and install them. Unfortunately, we can't guide you through this exact process as each device is different, but you should be able to find a guide specific to your hardware over on YouTube easily enough.

Update Critical Hardware Drivers

Drivers for storage controllers, chipsets, network adapters, and graphics can block the upgrade if outdated. You can update them through the manufacturer's website, Windows Update’s Optional Updates section, or a trusted driver management tool.

Note: After updating, restart and attempt the upgrade again.

Use the SFC and DISM Corruption Fixers

It's time to crack out the specialty tools! These two tools, when combined, will scan, check, and fix any minor corruptions and missing files that may be causing update issues. They'll take a little bit of time to run, but they usually give great results.

• Type cmd in Windows search, right-click on Command Prompt, and select Run as administrator.
• When UAC shows up, click Yes.
• Copy and paste the following command, pressing Enter after:

• Wait till the scan is finished. If Windows finds system file integrity violations, it will attempt to fix them. After this, use the set of the following commands, pressing Enter each time:

Use the Windows 11 Installation Assistant in Safe Mode

• Open Settings, then System, then Recovery, and use Advanced Startup to reboot into Startup Settings.
• Select Enable Safe Mode with networking.
• Run the Windows 11 Installation Assistant to perform the upgrade from this minimal state.

Run the Windows Update Troubleshooter

• Open the Get Help application as administrator.
• Search for Windows Update Troubleshooter and run the guided repair process.
• Follow each step until the tool identifies and resolves the issue or suggests contacting Microsoft Support.

Remove Peripheral Devices

External drives, printers, docking stations, and USB hubs can interfere with installation phases. So go ahead and disconnect all unnecessary peripherals before upgrading. Just leave your keyboard, mouse and WiFi device if you use one.

Free Up Disk Space

Make sure that you have at least 25 to 30 GB of free space. Remove temporary files, uninstall unused applications, and empty the recycle bin. If you don't have enough space for updates and upgrades, Windows 11 will complain. Usually, it will tell you the exact issue, but not all of the time.

Disable or Uninstall Third-Party Antivirus

Security software may block boot-phase operations.
Temporarily disable or uninstall it before upgrading.

Reset Windows Update Components

Stop Windows Update services, clear the SoftwareDistribution and Catroot2 folders, then restart the services.
This resolves corrupted update files.

Check for Pending Updates or Failed Rollbacks

Open Windows Update history and confirm there are no incomplete installations that may trigger SYSPREP issues. If you aren't sure how to roll back an update, you can check out the guide below.

How to uninstall a bad Windows 11 update. Roll back a Windows 11 update.