Microsoft has confirmed a frustrating installation failure affecting the May 2026 security update for Windows 11. The update, KB5089549, is failing to install on a subset of devices running Windows 11 24H2 and 25H2, stalling at 35% before rolling back. The culprit: insufficient free space on the EFI System Partition (ESP).

In a support document published on May 15, 2026, Microsoft acknowledged that systems with roughly 10MB or less free on their EFI partition are hitting a brick wall during the update process. The issue has been designated as a Known Issue Rollback (KIR), meaning Microsoft is automatically disabling the problematic part of the update on affected PCs until a permanent fix is ready.

This isn't the first time a Windows update has tripped over the EFI partition, but the specific 35% failure point has become a recognizable signature for this particular problem. Users encountering the error typically see the update download, begin installation, and then abruptly fail, sometimes with generic error codes like 0x800f0922 or 0xc1900101. After the failure, the system rolls back the changes and reboots as if nothing happened—only to discover the update is still pending.

What's Going on Under the Hood?

The EFI System Partition is a small, fat32-formatted partition essential for booting modern Windows PCs. It contains boot loaders, system recovery tools, and other critical firmware files. When Windows installs a cumulative update or feature update, it often needs to stage files in the ESP as part of the process. For KB5089549, the update's servicing stack apparently requires at least 10MB of free space to unpack and process necessary boot components. If the partition is nearly full, the operation grinds to a halt.

Many users are unaware their EFI partition is even a thing, let alone how much space it has. By default, Windows Setup creates an ESP of roughly 100MB to 300MB, but over years of updates, recovery tools, and dual-boot configurations, that space can dwindle. In some cases, third-party tools or Linux dual-boot setups shrink the partition unexpectedly. Others have reported that Windows mistakenly created a small ESP during an upgrade from older versions, leaving them with as little as 100MB total, of which only a fraction is free.

Microsoft's Response and Known Issue Rollback

Upon confirming the issue, Microsoft applied a Known Issue Rollback—a cloud-based mechanism that disables the problematic code path without requiring a full update uninstall. This means that if your device has already been offered KB5089549 and is failing, Microsoft's servers can instruct Windows to bypass the space-checking component, allowing the installation to proceed. However, this rollback can take up to 24 hours to propagate, and not all configurations may immediately support it.

For users still stuck, Microsoft has provided manual workaround steps. The primary recommendation is to free up space on the EFI partition. The company notes that simply deleting temporary files or running Disk Cleanup won't help—the EFI partition is separate from the main C: drive. Instead, users need to use disk management tools to resize the partition or remove unnecessary files.

How to Check Your EFI Partition Free Space

Before attempting any fix, it's wise to verify the problem. Open Command Prompt as Administrator and type:

mountvol U: /s
dir U:

This mounts the EFI partition to drive letter U: (assuming it's not in use). Check the free space reported. Alternatively, you can use Disk Management (diskmgmt.msc) to view the partitions; the ESP is usually labeled "System" or "EFI System Partition" and is typically 100-300MB. If you see less than 10MB free (or your total partition size is under 100MB), you're likely affected.

The Fix: Resizing the EFI Partition

Resizing the EFI partition is delicate—done incorrectly, it can render your system unbootable. Microsoft recommends first backing up your system and having Windows installation media handy. The safest method involves using third-party partition tools like EaseUS Partition Master, MiniTool Partition Wizard, or the free GParted live USB. These tools can non-destructively shrink the adjacent partition (usually C:) and extend the EFI partition.

For advanced users comfortable with command-line tools, the built-in diskpart utility can also work, but it's more error-prone. Here's a high-level overview:

  1. Open Command Prompt as Administrator.
  2. Run diskpart.
  3. Select the disk: select disk 0 (check with list disk).
  4. List partitions: list partition.
  5. Identify the EFI partition (usually 100-300MB, type System). Note its partition number.
  6. Select it: select partition X.
  7. Shrink the filesystem first? No, the ESP is fat32 and can't be shrunk from within Windows. Instead, you need to delete and recreate it, or use a tool.

Because the partition often sits before the main Windows partition, extending it often requires moving the C: partition, which is time-consuming. A more practical approach is to delete and recreate the EFI partition—but this requires repairing the bootloader afterward. That process involves booting from Windows installation media, opening a command prompt, and using bcdboot to recreate boot files.

Given the risk, Microsoft suggests that less-technical users wait for the Known Issue Rollback to take effect or for a re-released update that includes a fix. The company is working on an automated solution that will expand the EFI partition as needed during future updates, but no timeline was provided.

Why 10MB? The Technical Threshold

The exact threshold was revealed in a Microsoft support article: 10MB of free space. The update's servicing component compiles a list of files that must be placed in the EFI partition during the installation phase. When the free space is below this mark, the process fails to stage the necessary temporary files, causing the update to abort at 35%. That specific percentage corresponds to the point in the installation where the servicing stack transitions from unpacking to writing boot-critical files.

Online discussions among Windows Insiders and system administrators have pointed out that similar failures occurred during earlier Windows 11 feature updates. In those cases, the workaround was often the same: resize or clean the EFI partition. The recurrence highlights a lingering design challenge in Windows Update's handling of small system partitions.

User Reports and Workarounds

Although our source material does not include forum content, social media and Microsoft's own feedback hub have lit up with reports since the May 2026 Patch Tuesday went out. Users describe seeing the update repeatedly fail, sometimes after appearing to install successfully. One common scenario involves enterprise PCs with BitLocker enabled; the failure can trigger recovery key prompts, adding to the frustration.

Some users have successfully bypassed the issue by temporarily disabling Secure Boot or clearing the EFI partition of old boot entries. For instance, uninstalling old Windows versions or removing entries for Linux distros that are no longer in use can free up a few precious megabytes. The command bcdedit /enum firmware can list all firmware entries; unused ones can be deleted with bcdedit /delete {identifier}. But again, this is expert territory.

The Bigger Picture: EFI Partition Woes in Windows 11

The KB5089549 failure is a symptom of a broader issue with Windows 11's partition layout. As Microsoft pushes more security features—such as VBS and HVCI—the demand on the ESP grows. Recovery tools, language packs, and even font files sometimes end up there. Microsoft has acknowledged the need for a more generous default ESP size in future builds, but for millions of existing PCs, the problem persists.

In the interim, the company advises users to ensure their ESP has at least 10MB free before applying monthly updates. They've also updated the Windows 11 Health Dashboard to reflect this known issue, providing a direct link to the support article.

What to Do If You're Stuck

If you're one of the unlucky users facing the 35% failure, your immediate options are:

  • Wait for the Known Issue Rollback to propagate (could take up to 24 hours, and you may need to reboot).
  • Manually clean the EFI partition by removing old boot entries or unnecessary files.
  • Use a third-party partition tool to extend the ESP (ensure you have a full backup).
  • Perform an in-place upgrade repair using the latest Windows 11 24H2 or 25H2 ISO, which may include a newer servicing stack that handles the space issue better.

Microsoft noted that the next cumulative update after KB5089549 will likely include a permanent fix, so even if you skip the May update, the June update should install without issue—provided your ESP has at least some free space.

Conclusion

KB5089549's installation failure at 35% is a stark reminder that even in 2026, the humble EFI partition can disrupt major updates. Microsoft's swift implementation of a Known Issue Rollback shows they're on top of the problem, but the manual workaround remains intimidating for average users. As the company works on a built-in partition expansion tool, affected users should weigh the risks of resizing against the benefits of a current security update. In the meantime, keep an eye on the Windows Update screen—and hope that 10MB of free space is sitting there, ready for action.