Microsoft’s latest Patch Tuesday rollout for Windows 11 has hit a significant snag. Security update KB5089549, delivered on May 12, 2026, triggers an 0x800f0922 error and boot failure on devices running versions 24H2 and 25H2 that have a cramped EFI System Partition (ESP). The company acknowledged the issue in its known‑issues list, leaving millions of users scrambling for a fix.

The May 2026 Patch Tuesday surprise

KB5089549 is a cumulative security update addressing multiple vulnerabilities in Windows 11 24H2 and the newly released 25H2 feature update. The package weighs in at around 650 MB and includes fixes for three zero‑day exploits, along with the usual smattering of remote‑code‑execution and elevation‑of‑privilege patches. According to the Microsoft Security Response Center, the update closes 68 CVEs, making it one of the more critical rollouts of the year.

But the update process itself introduces a destructive bug. When the installation reaches the “Working on updates – 30%” stage, the PC automatically reboots into the Windows Recovery Environment (WinRE) to finalize the Safe OS phase. On systems where the EFI system partition lacks sufficient free space, the process halts with error code 0x800f0922 and the message “We couldn’t complete the updates. Undoing changes.” The device then enters a boot loop, repeatedly attempting and failing to complete the installation.

0x800f0922: a disk‑space error with a twist

The error code 0x800f0922 is familiar to veteran Windows administrators. It traditionally signifies that the system reserved partition (SRP) does not have enough free space to stage update files. In past occurrences, such as KB5034441 in early 2024, the culprit was a lack of space on the Windows Recovery partition. But KB5089549 exposes a different bottleneck.

This time, it is the EFI System Partition (ESP) that is starving. The ESP is a mandatory FAT32 partition that stores boot loaders, device drivers, and critical system files needed before Windows boots. Many OEMs still ship PCs with an ESP of only 100 MB, the absolute minimum size allowed by the Unified Extensible Firmware Interface (UEFI) specification. When the Safe OS update phase executes, it needs to copy a temporary boot manager and a signed update package onto the ESP. If available space on the ESP falls below roughly 15 MB, the operation fails and Windows rolls back.

Microsoft’s documentation confirms the threshold: “To successfully apply this update, the EFI system partition must have at least 15 MB of free space.” In real‑world testing by Windows Insiders and IT pros, the actual requirement can be slightly higher (up to 30 MB) if additional language packs or OEM customizations are present. Because most 100 MB ESPs already hold 80–95 MB of data, they fall short.

Affected versions and devices

The problem affects all editions of Windows 11, version 24H2 and 25H2, including Home, Pro, Enterprise, and Education SKUs. Thin clients and some budget laptops from Dell, HP, and Lenovo appear over‑represented in community reports, likely because they use minimal 100 MB ESP partitions. Custom‑built PCs where the user manually partitioned the drive with a small ESP are also at risk.

The 25H2 feature update is still fresh, having launched only three weeks earlier. Many users who installed it via Windows Update or the Installation Assistant now face this roadblock with their first cumulative patch. “I upgraded to 25H2 without issues, but now KB5089549 bricks my machine,” wrote one moderator on the Windows 11 subreddit. “Rollback works, but the update keeps re‑offering itself and failing again.”

User experiences and fallout

On the Microsoft community forums and Reddit, hundreds of threads have appeared since May 12. The pattern is consistent: the update downloads and starts to install; the PC restarts; the “Undoing changes” screen appears; and the device returns to the desktop with KB5089549 listed as a failed update. Some users report that the update tries to install repeatedly, leading to multiple failed boot cycles per day.

“I spent three hours trying to get past this error,” said a user on the TenForums site. “I ran DISM, sfc /scannow, cleared SoftwareDistribution, even reset Windows Update components. Nothing helped until I finally resized the EFI partition.”

Enterprise IT departments are particularly vocal. In a thread on the PatchManagement mailing list, system administrators noted that automated deployment rings for the May security update halted after the first wave of devices reported failures. Many have paused the rollout through Windows Update for Business or their WSUS configurations until a reliable workaround—or a revised update—becomes available.

A particularly acute pain point is that the error appears late in the installation process. Because the download and pre‑reboot phases consume significant bandwidth and processor time, the subsequent rollback wastes resources and leaves the device unprotected for an extended period.

Microsoft’s acknowledgment and current guidance

On May 13, Microsoft updated the known‑issue entry for Windows 11 24H2 and 25H2 to reflect the problem. The notice reads: “After installing KB5089549, some devices might report 0x800f0922. This issue occurs on systems where the EFI system partition has less than 15 MB of free space.” The company recommends that affected users manually resize the ESP using the diskpart command‑line tool.

Microsoft has not published an automatic fix. The Safe OS Dynamic Update mechanism, which occasionally expands partitions during major upgrades, is not triggered by a cumulative update like KB5089549. That leaves do‑it‑yourself partition resizing as the only immediate recourse.

There is no mention of a pending revised update that would detect the space constraint and gracefully bypass the Safe OS phase. In similar past incidents, Microsoft eventually issued an updated boot manager that compressed the required files, but that process took several weeks. For now, users must either resize the ESP or hide the update until a permanent fix ships.

How to resize the EFI System Partition (the hard way)

Resizing the ESP is possible but risky. The partition is critical for boot, and a mistake can render the system unbootable. Microsoft’s official instructions involve booting from a Windows installation USB, launching the command prompt, and using diskpart. The procedure is not scriptable in a way that avoids risks, so users must follow it carefully.

Here are the high‑level steps:

  1. Back up all important data. Partition modifications can corrupt the file system if interrupted.
  2. Boot from Windows 11 installation media. Select “Repair your computer” > “Troubleshoot” > “Command Prompt”.
  3. Identify the current partition layout. Type diskpart and then list disk, select disk N, list partition. Note the number and size of the ESP and the Windows partition.
  4. Shrink the Windows partition by 200–300 MB. Use the command select partition X (where X is the Windows or data partition next to the ESP), then shrink desired=300. If the partition is BitLocker‑encrypted, suspend protection first or use a third‑party tool capable of handling encrypted volumes.
  5. Expand the EFI partition. The ESP is encrypted with BitLocker? Actually, the ESP is FAT32 and cannot be encrypted by BitLocker, but the Windows partition may be. The ESP must be deleted and recreated larger, because diskpart cannot directly extend a FAT32 partition that is not the last partition in the layout. Utilities like GParted or MiniTool Partition Wizard can resize FAT32 partitions non‑destructively, but they require bootable media.
  6. Alternative: delete and recreate the ESP. This is far more dangerous. It requires backing up the contents of the ESP (typically the EFI\Microsoft\Boot folder), deleting the partition, creating a new larger one, formatting it FAT32, and restoring the boot files using bcdboot C:\Windows /s S: /f UEFI where S: is the newly assigned drive letter. This method works but demands absolute precision.

Experts universally recommend using a third‑party bootable partition manager like AOMEI Partition Assistant or EaseUS Partition Master, which can resize the ESP without manual file backup. These tools graphically shift the adjacent partition and expand the ESP into the freed space. Even then, creating a full disk image beforehand is prudent.

After expanding the ESP to at least 200 MB (a safe future‑proof size), the KB5089549 update installs without further issues. IT administrators who manage fleets can preemptively create a custom PowerShell script that checks ESP free space via Get‑Partition and flags devices needing attention before they deploy the update.

The lingering EFI partition problem

Why are devices still shipping with 100 MB ESPs in 2026? The UEFI specification from 2005 advised that 100 MB is sufficient for a minimal setup, but modern Windows installations routinely chew up 60–80 MB with language packs, firmware updates, and multiple boot entries. Dual‑boot Linux systems exacerbate the issue because each OS vendor adds its own boot loader.

OEMs are slow to adapt. A 200 MB ESP costs only a few megabytes of storage, but re‑imaging factory processes and coordinating with firmware teams require effort. Microsoft has periodically raised the bar—the Windows Hardware Compatibility Program for Windows 11 requires at least a 200 MB ESP for new designs, but that rule only applies to devices sold after mid‑2024. The enormous installed base of pre‑2024 Windows 11 PCs, as well as many “Windows 11 ready” devices that were originally shipped with Windows 10 and later upgraded, still carry the minimal partition.

This is not the first time the ESP has bitten an update. In 2023, KB5022913 also failed on some devices for the same reason, though the error code was different. Each instance reignites the debate about whether Microsoft should resize the ESP automatically during major upgrades. The Safe OS Dynamic Update introduced in Windows 10 version 2004 can grow the recovery partition but does not touch the ESP, leaving a critical gap.

What happens next?

Microsoft’s update health dashboard will likely be updated with a more refined detection algorithm in the coming weeks—flagging devices with insufficient ESP free space before offering KB5089549. However, that does not solve the underlying problem; it only prevents the update from being downloaded. Users would still need to resize the partition to receive future cumulative updates, which will inevitably grow in size.

A more lasting solution would be for Microsoft to ship a specialized boot manager stub that requires less space. The Windows Servicing team has used this approach before when the recovery partition overflowed. Alternatively, the company could re‑architect the Safe OS phase to use a temporary file on the main Windows partition instead of the ESP, though that introduces trust and integrity verification complexities.

For now, the burden rests on users and IT administrators. Check your ESP free space by running Get-Partition -DiskNumber $Disk | Where-Object {$_.GptType -eq '{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}'} | Select * in PowerShell. If the free space is below 30 MB, consider resizing before the next Patch Tuesday.

Windows 11 24H2 and 25H2 remain among the most secure consumer operating systems, but update delivery continues to be their Achilles’ heel when low‑level disk architecture collides with modern servicing demands.