Microsoft dropped its latest optional cumulative update for Windows 11 on May 26, 2026, and it comes with a warning that’s all too familiar: insufficient space on the EFI system partition can block installation of the prior month’s security patch. The preview update, tagged KB5089573, bumps Windows 11 25H2 to build 26200.8524 and 24H2 to 26100.8524. While it delivers the customary collection of non‑security fixes and quality enhancements, the release notes also document a hangover from the May 2026 Patch Tuesday update, KB5089549. On devices where the EFI system partition (ESP) is stuffed to the brim, that security update spits out error 0x800f0922 and refuses to install.

What’s inside KB5089573?

KB5089573 is what Microsoft calls a “C” release—an optional, non‑security preview that lets adventurous users and IT admins test the waters before the changes roll into the next mandatory Patch Tuesday package. These updates are offered through Windows Update and the Microsoft Update Catalog, but only if you actively check for updates. The build numbers, 26200.8524 and 26100.8524, represent minor ticks upward from the previous previews. Microsoft’s changelogs for such updates usually enumerate fixes for UI glitches, driver compatibility improvements, and under‑the‑hood reliability tweaks. This time, however, the most prominent item is the acknowledgement of a lingering installation problem affecting the security update that preceded it.

The update itself did not introduce new features; preview updates rarely do. Instead, it’s the kind of maintenance release that aims to smooth out rough edges before they hit the broader user base. For enterprises that rely on phased deployments, this is a chance to validate application compatibility and catch any regressions. The real headline, though, is the known issue that Microsoft chose to highlight.

The EFI system partition: a tiny giant

To understand why this error matters, you need to know what the EFI system partition does. On modern PCs that boot via UEFI firmware, the ESP is a small, FAT32‑formatted partition that holds the Windows Boot Manager, boot configuration data, and sometimes recovery tools or firmware update capsules. Windows Setup typically creates an ESP of 100 MB to 500 MB, depending on the disk layout and whether the installation was clean or an upgrade. That doesn’t sound like much, and in normal operation it’s plenty—until a cumulative update needs to stage temporary files there.

During Windows Update, the servicing stack sometimes uses the ESP as a scratch area. If the partition has little free space—commonly because of multiple boot entries, language packs, or legacy firmware blobs—the staging operation can fail. That’s exactly what happens with KB5089549. Microsoft’s documentation states that devices “with insufficient free space on the EFI system partition” are vulnerable to the 0x800f0922 error.

This isn’t a new headache. Similar issues dogged Windows 10 updates like KB5034441 in early 2024, where Microsoft eventually released a script to help users shrink the recovery partition and extend the ESP. The recurrence underlines a design tension: the ESP is treated as a fixed, hands‑off area, but modern servicing demands more breathing room than many OEM‑provisioned partitions provide.

Decoding error 0x800f0922

The hexadecimal error code 0x800f0922 has appeared in numerous Windows Update logs over the years. In generic terms, it signals CBS_E_INSTALLERS_FAILED or WU_E_TIME_OUT, but the underlying cause is often disk space exhaustion. When tied to the EFI partition, the scenario is straightforward: the update attempts to write temporary files, hits a wall, and rolls back. The affected system might show the update as “pending restart” or “failed” in Windows Update history, only to fail again after a reboot.

What makes this particular instance notable is that it’s a security update—KB5089549—that fails. Security updates are usually more urgent, and having them blocked by a full ESP can leave machines exposed until the issue is remedied. Microsoft’s advisory on the known issue doesn’t mention any automatic fix being pushed via the Known Issue Rollback (KIR) mechanism, which suggests the problem is environmental rather than a code defect that KIR can simply toggle off.

Users who encounter 0x800f0922 often see a message like: “Some update files are missing or have problems. We’ll try to download the update again later.” The error can also manifest as a cryptic installation failure in the Event Viewer. In this case, the root cause is explicitly documented, so the diagnostic path is clearer than it often is with random update failures.

Checking your EFI partition health

If you’re staring at a failed KB5089549 installation, your first step should be to inspect the EFI system partition. Windows doesn’t assign a drive letter to the ESP by default, but you can peek at it with the Disk Management console or the diskpart command‑line tool. Here is a quick way:

  1. Open a Command Prompt as Administrator.
  2. Run diskpart.
  3. Type list disk, then select disk X (where X is your system disk).
  4. Type list partition. The EFI partition is usually labeled “System” and is between 100 MB and 500 MB.
  5. To see its size and free space, assign it a temporary drive letter: select partition Y (the EFI partition number), then assign letter=S.
  6. Open File Explorer, right‑click the S: drive, and check properties.

If the free space is under 15‑20 MB, you’ve found the culprit. The exact threshold required by KB5089549 isn’t published, but updates have historically demanded anywhere from 15 MB to 50 MB of temporary space. A nearly full ESP is a ticking time bomb for any future update that touches the boot environment.

How to free up EFI partition space

Clearing space on the ESP is a delicate operation. Unlike conventional data drives, the ESP contains critical boot files, and accidentally deleting the wrong thing can render the system unbootable. That said, here are the strategies that have worked for past updates:

  • Remove unnecessary boot entries. If you’ve experimented with multiple operating systems or have EFI boot entries pointing to old Windows installations, those can be cleaned with the bcdedit command or by deleting the corresponding folders inside EFI\\ on the ESP. Extreme caution is required; only delete folders you are certain belong to unused OS installations.
  • Clean up language pack files. Some language resources are staged on the ESP. If you’ve installed and removed many language packs, remnants might linger. Microsoft’s own cleanup tools sometimes target these.
  • Delete recovery logs. The ESP occasionally accumulates log files from firmware capsule updates. Check the root and any hidden folders, but again, verify before deleting.
  • Resize the partition. This is the nuclear option. It involves shrinking the adjacent partition—usually the recovery partition or the main Windows partition—and using a tool like GParted Live or the built‑in diskpart (which cannot directly resize the ESP) to extend the EFI partition. Many users have successfully followed third‑party guides for this, but one misstep can leave you needing a full system image recovery.

Microsoft has not, as of the time of writing, released a dedicated fixer tool for this specific KB pairing. In past incidents, the company eventually published a PowerShell script or an automated recovery partition adjustment. Until such a tool arrives, manual intervention is the only recourse.

The broader update landscape

KB5089573’s spotlight on the EFI partition problem underscores a recurring theme in Windows maintenance: the interplay between decade‑old partition layouts and modern servicing stacks. When you buy a PC from a major OEM, the factory image often includes a 100‑MB EFI partition—the minimum size that Windows Setup creates if it detects a clean disk. That size was sufficient when boot managers were simple and firmware updates were infrequent. Fast‑forward to 2026, and the ESP might hold not just the Windows bootloader but also firmware capsules from multiple UEFI firmwares, recovery tools, and even diagnostic utilities from Lenovo, Dell, or HP.

Microsoft’s servicing strategy has also evolved. Cumulative updates are now monolithic, and they occasionally carry servicing stack updates that alter how files are staged. A single update may need to temporarily expand multiple megabytes of files on the ESP, and if that pushes the partition over the limit, you see errors like 0x800f0922.

The company’s decision to document the issue in the KB5089573 release notes is a double‑edged sword. It gives transparency, but it also confirms that this is a known hang‑up without an immediate, one‑click fix. For IT departments managing fleets, this is a call to audit EFI partition sizes before deploying the May 2026 security updates broadly.

What should you do if you’re affected?

First, do not panic. Error 0x800f0922 generally does not corrupt the system; it just prevents the update from installing. Your machine will continue to function normally, though it will miss the security patches that KB5089549 delivers. If you’re comfortable with disk management, the manual cleanup steps above can get you back on track. If not, you have a few options:

  • Wait for Microsoft to release guidance or a tool. The history of such issues suggests that a resolution often arrives within a month or two.
  • Consult your PC manufacturer. Some vendors provide their own disk‑repartitioning utilities that can safely expand the ESP without data loss.
  • Consider a clean installation of Windows 11 that lets you specify a larger EFI partition (500 MB is a safe choice for future‑proofing). This is drastic, but it eliminates the problem permanently.

One point of confusion: KB5089573 itself is an optional preview. If you haven’t installed it, you might still encounter the KB5089549 installation failure independently. In fact, the known issue applies to KB5089549, not to the preview update. The preview update simply documents the problem and lets you prepare. Microsoft’s wording suggests that the error occurs when you attempt to install the security update, regardless of whether you’ve taken the preview. So, delaying the preview won’t help; the solution lies in managing your EFI partition.

Forward‑looking thoughts

The Windows engineering team is undoubtedly aware that small EFI partitions are a lingering source of update friction. With each major feature update, Windows 11 adjusts its disk footprint, and the servicing stack becomes more sophisticated. A permanent fix might involve giving Windows Update the ability to dynamically resize the ESP when needed—something that requires deep integration with the UEFI firmware. That’s a heavy lift, but as the operating system continues to evolve, the pressure to automate this will only grow.

In the short term, users and admins should stay tuned to the Microsoft Known Issues page and the Windows Release Health dashboard for any mitigations. The company sometimes applies a Known Issue Rollback that temporarily disables the problematic part of the update until the environment is ready, but there’s no indication yet that KIR can be used for a space‑related issue.

If history is any guide, we’ll see a revised version of KB5089549 that either reduces its ESP footprint or runs a pre‑flight check to warn users before the installation. For now, the most prudent step is to check your EFI partition and make room if it’s dangerously full.

Conclusion

KB5089573 and the accompanying EFI partition warning are a stark reminder that even routine updates can be tripped up by low‑level disk geometry. The May 2026 security patch remains critically important, but for some users it will be out of reach until the ESP gets a bit of breathing room. Whether you choose to manually clean up or hold out for an official fix, awareness of the issue is the first line of defense. Keep an eye on your partitions—they might be small, but they can cause big headaches.