Microsoft shipped the optional KB5089573 preview cumulative update for Windows 11 on May 26, 2026, delivering a targeted fix for a nagging installation bug that has been tripping up users with specific EFI partition configurations. The update pushes Windows 11 version 25H2 systems to OS build 26200.8524 and version 24H2 machines to build 26100.8524. While the patch bundles the usual assortment of quality improvements, its headline change addresses error 0x800f0922, a failure that blocks cumulative update installation when the EFI system partition (ESP) is misreported or underprovisioned. System administrators and enthusiasts who manually partitioned their drives or cloned smaller EFI volumes have been stuck for weeks; this fix finally clears the logjam.
The 0x800f0922 Error and EFI Partition Woes
Error 0x800f0922 typically signals that the servicing stack is unable to complete an update operation due to insufficient space or incorrect partition attributes on the system drive. In the case of Windows 11 cumulative updates, the culprit is almost always the EFI system partition—the small, FAT32‑formatted partition that holds the bootloader and recovery tools. During update installation, the servicing engine stages a fresh copy of the Windows boot manager and related files on the ESP. If the ESP is smaller than 100 MB, formatted with a non‑standard cluster size, or marked with unexpected GPT attributes, the staging fails and throws the 0x800f0922 error.
The problem gained widespread attention in May 2026 after several monthly cumulative updates refused to install on systems where the ESP was created by third‑party partitioning tools, manually resized, or cloned from an older NVMe drive. In many cases, the partition was perfectly functional for daily booting but lacked the free space or correct descriptor flags that the update engine demands. Users saw the update fail at around 96% completion, forcing a rollback. The only workaround—manually resizing the ESP using diskpart or third‑party tools—was risky for novice users and occasionally led to unbootable systems.
What KB5089573 Delivers
KB5089573 introduces a dual‑phase remediation. First, the update engine now performs a pre‑installation check that inspects the ESP for minimum size (100 MB) and correct GPT attributes. If the partition fails validation, the engine invokes a lightweight repair tool that attempts to non‑destructively extend the ESP by reclaiming unused space from the adjacent Microsoft Reserved Partition (MSR) or primary system partition. This operation runs before the real update payload lands, so no user data is touched. Second, the update relaxes the staging logic to handle ESPs formatted with cluster sizes up to 8 KB, accommodating more exotic setups.
The repair tool, dubbed efipartfix.dll, works entirely in the Windows Preinstallation Environment (WinPE) during the reboot cycle. It shrinks the MSR by a few megabytes and grows the ESP accordingly, then updates the partition table and synchronizes the Boot Configuration Data (BCD) store. If adjacent free space is unavailable, the tool aborts cleanly with a clear error message in the update history, advising the user to manually shrink the C: drive and extend the ESP. Microsoft claims the automated fix succeeds on over 95% of affected configurations.
Additional Fixes and Quality Improvements
Beyond the EFI fix, KB5089573 rolls up numerous non‑security fixes that have been incubating in the Beta and Release Preview channels. A long‑standing issue where the Windows Search indexer would crash when scanning UTF‑8 encoded file paths on exFAT volumes has been resolved. The update also patches a memory leak in the Local Security Authority Subsystem Service (LSASS) triggered by repeated Kerberos authentication failures on domain‑joined machines—a bug that forced administrators to schedule weekly reboots on heavily loaded terminal servers. File Explorer gains a reliability improvement for the context menu; it no longer freezes for several seconds when right‑clicking a disk drive with over 500 folder entries at the root level.
Graphics and audio get attention too. A race condition in the Windows Display Driver Model (WDDM) that caused a black screen when hot‑plugging a DisplayPort 2.1 monitor while playing protected video content has been eliminated. The Bluetooth audio stack now correctly enumerates LE Audio headsets after a fast startup boot, fixing a regression introduced in last month’s security update. Additionally, the Network and Internet settings applet no longer crashes when toggling the “Random hardware addresses” switch for Wi‑Fi networks with hidden SSIDs.
Known Issues in This Build
No update is flawless, and KB5089573 carries a short but notable list of known issues. Microsoft warns that systems with the Windows Subsystem for Linux (WSL) 2 installed may experience a 30‑second delay during shutdown if the WSL virtual machine is running an Ubuntu 26.04 instance with systemd enabled. A fix is expected in a future WSL kernel update. Enterprise customers using the Windows Defender Application Guard (WDAG) extension for Microsoft Edge should note that the extension may fail to activate after installing this update; a manual re‑registration via PowerShell (Add‑AppxPackage -Register) resolves it.
More seriously, certain Dell and Lenovo laptops equipped with the Realtek USB4 Host Router may lose Thunderbolt 4 connectivity after resuming from modern standby. The issue is not specific to KB5089573 but appears to be amplified by it. Work is underway with OEMs to release firmware updates. In the meantime, affected users can disable modern standby via powercfg /setacvalueindex scheme_current sub_video videoconfig 0 as a temporary mitigation.
How to Get KB5089573
As an optional preview update, KB5089573 is not pushed automatically through Windows Update. Users must manually check for updates and select the “Download and install” link under the optional updates section. Enterprise customers can obtain the update from Microsoft Update Catalog, Windows Server Update Services (WSUS), or Configuration Manager with the latest branch settings. The MSU standalone installer is available for x64, ARM64, and x86 platforms under KB articles 5089573 (25H2) and 5089574 (24H2).
Installation requires at least 5 GB of free disk space on the system drive and 1 GB on the EFI partition. The update will fail with error 0x80070070 if these prerequisites are not met. After the update, the PC must be restarted; a full shutdown and cold boot is recommended to ensure the EFI partition adjustments take effect.
Community Reaction and Early Feedback
In the first 24 hours after release, feedback on Windows forums and Reddit has been largely positive. Users with previously stuck systems confirmed that the error 0x800f0922 disappeared and the monthly cumulative update installed normally. One sysadmin who manages a fleet of 200 Dell Latitude 7440 laptops reported a 100% success rate after deploying the update via WSUS, noting that the automatic ESP repair saved an estimated 40 hours of manual intervention.
However, a vocal minority reported edge cases. Users with dual‑boot Linux setups where GRUB had claimed the ESP saw the repair tool fail because the ESP was flagged as a Linux ESP GUID. Microsoft’s documented workaround involves changing the GUID to the Windows standard before retrying. Others whose ESP was the very first partition (offset 0) but smaller than 100 MB encountered a cryptic “Failed to allocate cluster” message; diskpart commands are provided in the KB article to reposition the partition.
Why This Fix Matters for the Future
The 0x800f0922 EFI issue is not entirely new—similar problems surfaced in Windows 10 Feature Updates five years ago—but its persistence highlights how fragile the servicing model can be when partition layouts deviate from Microsoft’s defaults. The KB5089573 approach of pre‑staging repair actions represents a more resilient pattern. Future updates are likely to adopt similar self‑healing mechanisms for other disk‑related blockages, potentially eliminating the need for the System Reserved partition entirely by migrating it into the ESP.
Microsoft’s swift response also underscores the growing importance of preview updates as a safety valve. Since the Windows 11 servicing pipeline shifted to a monthly cadence with occasional on‑demand releases, these optional patches serve as a real‑world test ground for fixes that later roll into the mandatory Patch Tuesday update. Early adopters who install KB5089573 now will effectively validate the fix for the broader user base, reducing the risk of widespread disruption when the June 2026 security update ships.
For organizations that blocked the May 2026 security release due to the installation bug, KB5089573 is a green light to resume patching. The cumulative update includes all security fixes from the previous month, so systems leapfrog directly to a protected state without exposing themselves to known vulnerabilities. Microsoft’s security response center has not assigned any CVEs to this update, classifying it as purely a quality reinforcement.
Step‑by‑Step: Verifying Your EFI Partition Readiness
If you’ve been avoiding updates due to error 0x800f0922, you can confirm that your system will accept KB5089573 by checking the ESP properties. Open an elevated PowerShell prompt and run:
Get-Partition -DiskNumber (Get-Disk -FriendlyName 'NVMe*' | Select-Object -First 1).Number | Where-Object GptType -eq '{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}' | Format-List Size,GptType
This command prints the size and GUID of the EFI partition on the first NVMe disk. The size should read 104,857,600 bytes or larger (100 MB). If it’s smaller, applying the update will trigger the repair. To watch the repair process live, you can boot into WinPE and monitor the efipartfix.log file generated in the C:\Windows\Logs\CBS directory.
A less technical option is to let the update fail once—the error code 0x800f0922 will appear in Windows Update settings. After that, KB5089573’s pre‑check will automatically kick in on the next attempt.
Conclusion
KB5089573 is not a blockbuster update aimed at changing the Windows experience; it is a targeted, almost surgical fix for a specific boot‑partition snafu that had been frustrating a niche but vocal segment of power users and IT pros. By pairing automated repair logic with relaxed tolerances, Microsoft has turned a source of helpdesk calls into a self‑resolving routine. The update’s broader quality improvements, especially the LSASS memory leak fix, will be welcomed by enterprise admins. As always, the wise approach is to test the update on a representative subset of devices before broad deployment, but for anyone still stuck on a frozen build, this update is the escape hatch they’ve been waiting for.