Microsoft Confirms Windows 11 Update Failures Linked to EFI Partition Space

Microsoft has acknowledged a new installation issue affecting the May 2026 Windows security update KB5089549 on certain Windows 11 systems.

According to Microsoft, the update may fail during installation on devices with insufficient free space in the EFI System Partition (ESP), generating error code 0x800f0922 during the reboot phase of the update process.

Update Fails During Reboot Phase

Microsoft says affected systems typically progress through the initial stages of the update before failing at roughly 35 to 36 percent completion during restart.

The system then rolls back the installation automatically while displaying the message:

“Something didn’t go as planned. Undoing changes.”

The issue specifically impacts Windows 11 versions 24H2 and 25H2, while Windows Server editions reportedly remain unaffected.

Microsoft identified systems with 10MB or less available space on the EFI partition as particularly vulnerable to the problem.

Log Files Reveal Storage-Related Errors

Affected systems may contain related error entries inside:

C:\Windows\Logs\CBS\CBS.log

Microsoft highlighted several common messages associated with the issue, including:

• “SpaceCheck: Insufficient free space”
• “ServicingBootFiles failed. Error = 0x70”
• “SpaceCheck: <value> used by third-party/OEM files outside of Microsoft boot directories”

The EFI System Partition is a small dedicated boot partition used by Windows during startup and update operations. Limited space within the partition can interfere with boot file servicing during major updates.

Microsoft Provides Two Temporary Workarounds

Microsoft currently recommends two separate mitigation methods while a permanent fix remains in development.

The safer option for most users involves using Known Issue Rollback (KIR), which restores the system to a previous working update state automatically.

More advanced users with administrator access can also apply a registry modification intended to bypass the ESP padding requirement during installation.

Microsoft provided the following command for affected systems:

reg add "HKLM\SYSTEM\CurrentControlSet\Control\Bfsvc" /v EspPaddingPercent /t REG_DWORD /d 0 /f

After running the command through an elevated Terminal or Command Prompt session, users should restart the PC and attempt the update installation again.

Permanent Fix Still in Development

Microsoft says it is actively working on a permanent resolution that will arrive through a future Windows update.

The incident adds to a growing number of update-related reliability issues affecting Windows 11 as Microsoft continues expanding feature updates and security servicing requirements across newer system builds.