A routine cumulative update for Windows 11 has turned into a headache for many users as Microsoft confirms installation failures tied to insufficient free space on the EFI System Partition (ESP). The May 2026 security update, KB5089549, can refuse to install on version 24H2 and 25H2 machines when the ESP has approximately 10 MB or less of free space, triggering the dreaded error code 0x800f0922. Users report being stuck in an endless loop of failed installations, with some systems automatically rolling back the changes after multiple unsuccessful attempts.

This problem isn't just limited to older hardware. Even relatively new PCs with small default ESP sizes — typically between 100 MB and 260 MB — can hit this limit if the partition contains accumulated logs, redundant font files, or leftover recovery data. Microsoft acknowledged the issue in mid-May 2026 and published a support document outlining both manual workarounds and a Known Issue Rollback (KIR) for affected systems.

Why Does KB5089549 Fail on Small EFI Partitions?

The EFI System Partition is a critical component of UEFI-based computers. It stores the Windows Boot Manager, boot configuration data, and other essential startup files. During major updates, the installation process needs to write new boot components and backup existing ones within the ESP. If the partition doesn't have enough breathing room, the update engine can't complete these tasks and throws error 0x800f0922.

KB5089549, a cumulative update released on May 12, 2026, includes security fixes, quality improvements, and servicing stack updates. For Windows 11 24H2 (OS Build 26000.xxxx) and 25H2 (OS Build 26100.xxxx), the update package carries a larger footprint in the ESP due to changes in Secure Boot handling and recovery partition reconfiguration. Microsoft's documentation now recommends at least 15–20 MB of free space on the ESP for seamless updates, but many factory-configured systems fall short.

Identifying the Problem: Symptoms and Error Codes

Users affected by this issue typically see the following pattern:

  • The update downloads normally but fails during installation at around 25%–35% progress.
  • Windows Update history displays error 0x800f0922.
  • Event Viewer logs under Application and Service Logs \\ Microsoft \\ Windows \\ WindowsUpdateClient show Event ID 20 with message “Update failed to install.”
  • The system may automatically revert to the previous build after a second failed attempt, prompting a “We couldn't complete the updates. Undoing changes” message.

If you run the Disk Management tool or a third-party partition utility, you can check the free space on the EFI System Partition. On a standard installation, the ESP is the first partition on the disk, formatted as FAT32, and often just 100 MB in size. If less than 10 MB remains available, you're almost certainly hitting this bug.

Microsoft's Response and Known Issue Rollback

Microsoft quickly added a Known Issue Rollback to the list of acknowledged problems for KB5089549. KIR is a mechanism that automatically reverts a problematic non-security fix to allow installations to complete. However, because the issue is related to ESP capacity rather than a specific code change, the rollback only addresses the failure symptom for a subset of configurations. Users still need to address the underlying space shortage to install future updates.

In its health dashboard, the company stated: \"After installing KB5089549, you might encounter error 0x800f0922 if your EFI system partition has insufficient free space. To mitigate this, ensure at least 20 MB of free space before retrying the installation.\" The notice also confirmed that affected Windows 11 editions include Home, Pro, Enterprise, and Education on 24H2 and 25H2 feature updates.

How to Fix It: Step-by-Step Workarounds

There are two proven ways to resolve the 0x800f0922 error on KB5089549. Both require administrator privileges and careful handling of disk partitions. Always back up your data and create a system restore point before making partition changes.

Method 1: Free Up Space on the Existing EFI Partition

This is the safer, non-destructive approach but is only effective if you can clear a few megabytes of non-essential files.

  1. Open Command Prompt as Administrator.
  2. Mount the EFI system partition by typing mountvol S: /S (this assigns drive letter S to the ESP).
  3. Navigate to S: using S: and explore its contents. Be extremely cautious — do not delete the EFI\Microsoft\Boot folder.
  4. Remove unnecessary language folders within EFI\Microsoft\Boot if present (e.g., fr-FR, es-ES). Only keep en-US if that’s your system language.
  5. Delete font files under EFI\Microsoft\Boot\Fonts except for the default ones (like segmono_boot.ttf, segoe_slboot.ttf, etc.).
  6. If present, delete redundant log files in the root or old .bin files from past updates.
  7. After clearing at least 5–10 MB, unmount with mountvol S: /D.

Be extremely conservative. Deleting the wrong file can render your system unbootable. If you’re unsure, skip to Method 2.

Method 2: Extend the EFI System Partition

When cleanup isn't enough, resizing the ESP is the surest fix. This process involves shrinking the adjacent partition (usually the C: drive) and allocating that space to the ESP. Because Windows built-in tools can't resize the ESP directly, you'll need a third-party partition manager. Free options include GParted Live (bootable USB) or MiniTool Partition Wizard. Follow these general steps:

  1. Create a bootable USB drive with a partition manager that supports moving/resizing the ESP.
  2. Boot from the USB drive. The partition layout may show a small FAT32 partition at the start of the disk.
  3. Shrink the partition immediately after the ESP (typically the Windows C: drive) by 50–100 MB.
  4. Move that free space adjacent to the ESP and then extend the ESP into that unallocated space.
  5. Apply changes and reboot into Windows.
  6. Check that the ESP now has at least 50 MB free; then retry the update.

If you’re uncomfortable with partition operations, seek assistance from a professional or consider clean-installing Windows with a manually specified larger ESP (e.g., 200–300 MB) using Windows Setup’s Shift+F10 command prompt and diskpart.

Preventing Future EFI Space Issues

Microsoft’s OEM partners have long been criticized for shipping devices with the bare minimum ESP size. While 100 MB is officially sufficient, real-world usage during feature updates demands more. For users who often install multiple language packs or use advanced boot configurations, a 200 MB ESP is a prudent choice.

If you built your own PC, you can set a custom ESP size during installation using diskpart. The command create partition efi size=300 before installing Windows ensures ample room. For existing systems, periodically check the ESP's free space using mountvol or third-party tools, especially before major semi-annual updates.

What to Do If the Update Is Already Stuck

If KB5089549 is stuck in a reboot loop and continually fails, force a rollback:

  • Boot into Advanced Startup Options (hold Shift while selecting Restart).
  • Navigate to Troubleshoot > Advanced Options > Uninstall Updates.
  • Choose “Uninstall latest quality update” and follow the prompts.
  • After rollback, pause updates for 7 days, fix the ESP space issue, and then manually re-download KB5089549.

Alternatively, use System Restore to return to a point before the update was attempted, assuming restore points are enabled.

Community Reactions and Workarounds

On Microsoft TechNet forums and Reddit, users shared mixed experiences. Some cleared space by deleting old MacOS dual-boot remnants from the ESP. Others resorted to imaging the partition, resizing it in a VM, and restoring. A few noted that simply updating drivers and running Disk Cleanup before retrying resolved the error on systems where the ESP had exactly 10 MB free — a borderline case.

Microsoft’s swift KIR deployment softened the impact, but the lack of an official in-place repair tool frustrated many. One IT admin wrote, “We have 200 machines failing this update. Group Policy disables KIR on some, so we’re stuck manually extending partitions — a nightmare.”

Looking Ahead

Microsoft has not yet announced plans to increase the default ESP size in Windows 11 OEM specifications. However, this incident may pressure hardware partners to adopt more forward-looking configurations. Future updates, including the rumored 25H2 refresh later this year, will likely include checks that pre-assess ESP free space and offer actionable guidance before failing.

In the meantime, users should treat EFI partition maintenance as routine PC hygiene — just as important as clearing temporary files. A few extra minutes spent ensuring 20–50 MB of free ESP headroom can save hours of troubleshooting when the next cumulative update drops.