Windows 11 KB5089549 EFI System Partition Full Fix Diskpart Guide

What is the KB5089549 EFI Partition Full Issue?

The EFI System Partition (ESP) is a small FAT32 partition (usually 100 MB on modern PCs, but only 100 MB or less on legacy installations) that stores the UEFI boot loader, the Windows Boot Manager (bootmgfw.efi), and locale files. Starting with the May 13, 2026 cumulative update (KB5089549), Windows requires at least 10 MB of free space inside the ESP to install the new servicing stack bootloader payload. PCs originally imaged before 2018 — when Microsoft's default ESP size was 100 MB — frequently fall below this threshold after years of cumulative bootloader rotations.

When the ESP is full, KB5089549 rolls back at 35–36% with error 0x800f0922 and a CBS_E_INSTALLERS_FAILED entry in C:\Windows\Logs\CBS\CBS.log. Microsoft confirmed the root cause on May 16, 2026 and began rolling out a Known Issue Rollback (KIR), but the underlying problem — an undersized ESP — only goes away by cleaning or resizing the partition.

When does it occur?

• KB5089549 fails at the post-reboot servicing screen with error 0x800f0922
• mountvol S: /S then dir S:\ shows less than 10 MB free
• The Windows installation predates 2018 (Build 1709 or earlier originally)
• The PC dual-boots with Linux, macOS, or uses rEFInd/Clover/GRUB
• OEM stored Winre.wim on the ESP instead of a dedicated recovery partition

Common causes

• ESP is only 100 MB and Windows has consumed it with each cumulative update
• Orphaned bootmgfw.efi.bak, BCD.LOG1, and BCD.LOG2 files left from prior updates
• Duplicate boot fonts under \EFI\Microsoft\Boot\Fonts\ (wgl4_boot.ttf, segoe_slboot.ttf)
• Third-party EFI loaders (rEFInd, Clover, GRUB, Ubuntu shim) consuming several MB each
• Winre.wim (the Windows Recovery Environment image) stored inside the ESP — adds ~500 MB on OEM-imaged Dell, HP, and Lenovo systems
• Stale BitLocker pre-boot authentication binaries from a removed configuration
• Stub \EFI\Boot\bootx64.efi copies from failed Windows feature update attempts

Step-by-step fixes

1. Inspect the ESP from diskpart. Open an elevated Command Prompt and run:

   diskpart
   list disk
   select disk 0
   list partition

Identify the partition labeled System — that is your ESP. Note its size (typically 100 MB).

1. Mount the ESP to a drive letter so you can browse it. Still inside diskpart:

   select partition 1
   assign letter=S
   exit

Replace partition 1 with the System partition number from step 1. Confirm with dir S:\EFI\Microsoft\Boot\.

1. Check free space precisely with PowerShell:

   Get-Volume -DriveLetter S | Select-Object SizeRemaining, Size

If SizeRemaining is below 10485760 bytes (10 MB), KB5089549 will refuse to install.

1. Delete safe, regeneratable files to reclaim space. Windows recreates these on next boot:

   Remove-Item "S:\EFI\Microsoft\Boot\Fonts\*.ttf" -Force
   Remove-Item "S:\EFI\Microsoft\Boot\*.bak" -Force
   Remove-Item "S:\EFI\Microsoft\Boot\BCD.LOG*" -Force
   Remove-Item "S:\EFI\Microsoft\Recovery\BCD.LOG*" -Force -ErrorAction SilentlyContinue

This usually frees 6–15 MB. Re-run Get-Volume to verify.

1. Remove orphaned third-party bootloader entries if you no longer dual-boot. From an elevated prompt: bcdedit /enum firmware lists every UEFI boot entry. Delete unused ones with bcdedit /delete {guid} and then remove their folders (S:\EFI\ubuntu\, S:\EFI\refind\, S:\EFI\clover\) using Remove-Item -Recurse -Force.

1. **Move Winre.wim off the ESP** if it lives there. Run reagentc /info to find its current path. If the output shows \\?\GLOBALROOT\device\harddisk0\partition1\Recovery\WindowsRE\Winre.wim, partition 1 is the ESP. Disable WinRE with reagentc /disable, move the file to C:\Recovery\WindowsRE\ (robocopy S:\Recovery\WindowsRE\ C:\Recovery\WindowsRE\ Winre.wim /MOVE), update with reagentc /setreimage /path C:\Recovery\WindowsRE, then reagentc /enable.

1. Resize the ESP with MiniTool Partition Wizard or AOMEI Partition Assistant if cleanup is not enough. Free editions support shrinking the C: partition by 200 MB and growing the adjacent ESP. Steps: shrink C: by 200 MB, move the unallocated space left of C: until it is adjacent to the ESP, then extend the ESP. Always create a system image backup first — a botched resize can leave the PC unbootable.

1. Retry the update. Unassign the temporary letter (mountvol S: /D), then Settings → Windows Update → Check for updates. KB5089549 should install past 36% and report Build 26100.4751 (24H2) or 26200.2310 (25H2) under winver.

If it still doesn't work

If KB5089549 still rolls back after the ESP has more than 10 MB free, the servicing stack itself is the bottleneck. Run DISM /Online /Cleanup-Image /AnalyzeComponentStore followed by DISM /Online /Cleanup-Image /StartComponentCleanup /ResetBase to compact C:\Windows\WinSxS\, then sfc /scannow to repair any tampered system files. If DISM reports 0x800f081f, mount the latest Windows 11 ISO and run DISM /Online /Cleanup-Image /RestoreHealth /Source:WIM:D:\sources\install.wim:1 /LimitAccess against it. For BitLocker-encrypted systems, suspend protection with manage-bde -protectors -disable C: -RebootCount 1 before retrying so the firmware does not prompt for the 48-digit recovery key after the ESP write completes.