UEFI and ESP Explained: Boot, Secure Boot, and Windows Recovery

EFI is the small, pre‑OS layer that brings your PC hardware to life and hands control to Windows, replacing the decades‑old BIOS with a faster, more secure, and far more flexible pre‑boot environment that modern Windows installs require.

Background / Overview​

The Extensible Firmware Interface (EFI) — now most commonly encountered as UEFI (Unified Extensible Firmware Interface) — is firmware that initializes hardware, locates bootloaders on a dedicated partition, and starts the operating system. Unlike legacy BIOS, UEFI is a standardized, extensible firmware platform that supports large drives, secure boot chains, richer firmware UIs, and 32‑/64‑bit pre‑boot code. These changes matter because they directly affect startup speed, platform security (Secure Boot + TPM attestation), and compatibility with modern NVMe SSDs and multi‑terabyte storage.
This article explains what EFI/UEFI is, how Windows uses the EFI System Partition (ESP), how to identify and safely manage that partition in Windows, and the most reliable ways to repair or rebuild UEFI boot files when startup fails. The technical claims below are cross‑checked against Microsoft’s official documentation and practical community recovery workflows to ensure accuracy.

---

What EFI / UEFI Actually Is​

The firmware layer that boots your PC​

At power‑on, UEFI firmware runs a hardware initialization sequence (CPU, memory, chipset, storage, and peripherals) and then searches for a boot manager on the EFI System Partition. That boot manager (for Windows, the Windows Boot Manager) loads the Windows kernel and passes control to the OS. Unlike BIOS, which depended on legacy boot sectors and a fixed MBR flow, UEFI boots by reading files from a FAT‑formatted ESP and supports multiple OS bootloaders coexisting in the same partition.

Why "UEFI" rather than "EFI"​

EFI was originally developed as Intel’s Extensible Firmware Interface; the specification evolved into UEFI under a consortium and is now the universal standard shipped on modern PCs. In practice, UEFI and EFI are used interchangeably in consumer documentation, but UEFI is the current, standardized implementation you’ll actually encounter on current hardware.

---

Key differences between legacy BIOS and UEFI​

• Boot speed: UEFI boots faster because it uses direct file loading from the ESP and supports parallel initialization in many firmware implementations.
• Drive support: BIOS+MBR historically limited usable disk space (~2 TB). UEFI+GPT supports modern multi‑TB drives and NVMe SSDs.
• Interface: BIOS used a text menu; UEFI supports mouse + graphical setups and richer firmware features.
• Architecture: BIOS ran 16‑bit code; UEFI supports 32/64‑bit pre‑boot code and additional drivers that can be loaded by firmware.
• Security: BIOS had little built‑in boot security; UEFI adds Secure Boot and works with TPM attestation to protect boot integrity.

---

How Windows uses EFI: the EFI System Partition (ESP)​

What the ESP contains​

The EFI System Partition is a small FAT32 partition that stores bootloaders, firmware drivers, and boot manager files for all operating systems on the machine. For Windows, the relevant files live under \EFI\Microsoft\Boot\ (for example, bootmgfw.efi) and the BCD store is kept on that partition. UEFI firmware mounts or reads the ESP at boot time to find eligible bootloaders.

Partition GUID and identification​

Under GPT, the ESP is identified by the standard GUID C12A7328‑F81F‑11D2‑BA4B‑00A0C93EC93B; that GUID is the canonical identifier used by Windows tools and partition utilities. If the partition’s GUID is wrong or its attributes aren’t set correctly, Windows and firmware may not recognize it as an ESP. Tools like DiskPart can set the correct ID when needed.

Typical size and practical recommendations​

Historically 100 MB ESPs were common, but modern Windows updates and some OEM tools now expect 100–300 MB or larger; many experts and Microsoft‑backed workflows recommend 260–500 MB for robustness, because feature updates and third‑party pre‑boot utilities occasionally need extra room. If Windows Setup or feature updates complain about insufficient ESP space, resizing to 300–500 MB is a common remedy.

---

How to check whether Windows is using UEFI (safe checks)​

Use built‑in Windows tools — these are read‑only checks and safe.

• msinfo32 method (System Information): Press Windows + R, type msinfo32 and press Enter. Look at BIOS Mode — “UEFI” means the system booted in UEFI; “Legacy” means BIOS mode. This is the simplest check.
• Disk Management / Partition style: Open Disk Management, right‑click the system disk → Properties → Volumes tab → Partition style. "GUID Partition Table (GPT)" indicates a GPT disk; to boot UEFI requires GPT for Windows.
• Boot loader file check (command line): Inspect BCD or boot file paths (for instance, presence of \EFI\Microsoft\Boot\bootmgfw.efi) to confirm UEFI boot paths are in use.

---

How to access UEFI settings from Windows​

You can enter firmware settings using Advanced Startup:

• Settings → System → Recovery → Advanced startup → Restart now.
• Choose Troubleshoot → Advanced options → UEFI Firmware Settings → Restart to enter firmware.

Most motherboards also accept a hotkey at boot (Del, F2, Esc, F12) — consult the vendor documentation for the exact key.

---

How to identify the EFI partition safely in Windows​

Before making changes, back up data. Use these read‑only or safe commands to locate the ESP.

1) Disk Management​

Open Disk Management (Win + X → Disk Management) and look for a small FAT32 partition labeled EFI System Partition, usually around 100–300 MB. Do not delete or reformat it unless you’re repairing or rebuilding with clear instructions.

2) DiskPart (Command Prompt as Administrator)​

• Open an elevated Command Prompt.
• Run:
• diskpart
• list disk
• select disk 0 (replace 0 with your system disk)
• list partition

Look for a small partition labeled System or EFI, typically 100–300 MB. If you need to mount it temporarily for inspection or to run recovery commands, assign a letter (careful to remove it afterward). Many community guides and recovery workflows use this technique when rebuilding boot files.

3) PowerShell (GUID filter)​

Open Windows Terminal (Admin) and run:
Get‑Partition | Where‑Object {$_.GptType -eq "{C12A7328-F81F-11D2-BA4B-00A0C93EC93B}"}
This filters partitions by the ESP GUID so you can reliably find it on GPT disks. Note the disk/volume number and any assigned drive letter.

---

When the EFI partition is missing or damaged — safe repair overview​

If Windows won’t boot because EFI files are missing, there are validated, commonly used recovery flows. Each has tradeoffs; pick the one that matches your level of risk tolerance and available backups.
Important safety rules first:

• Create a full disk image if possible (Macrium, Acronis, or similar).
• Suspend BitLocker before making partition changes (if BitLocker is active).
• Double‑check disk numbers — selecting the wrong disk in diskpart destroys data.

A. Try the standard boot repair + bcdboot (WinRE / install media)​

• Boot a Windows install USB → Repair your computer → Troubleshoot → Command Prompt.
• Identify the ESP and assign it a letter with diskpart.
• Run:
• bcdboot C:\Windows /s S: /f UEFI

This copies Windows boot files to the ESP and registers a Windows Boot Manager entry. This is the first‑line and least invasive approach.

B. Recreate a missing ESP manually (DiskPart + bcdboot)​

If the ESP has been deleted or corrupted and you have disk space:

• Boot into WinRE and open an elevated Command Prompt.
• Use diskpart to create a FAT32 EFI partition:
• diskpart
• select disk X
• create partition efi size=260
• format fs=fat32 quick
• assign letter=S
• exit
• Run:
• bcdboot C:\Windows /s S: /f UEFI

That creates the ESP and installs the UEFI boot files. Community recovery threads widely use this approach and document the exact commands.

C. Convert MBR→GPT and create an ESP (when moving from Legacy BIOS to UEFI)​

If Windows was installed in legacy MBR mode and you need UEFI (for example, to enable Secure Boot and TPM support), Microsoft’s MBR2GPT is the supported conversion tool that can non‑destructively convert a system disk to GPT and create an ESP when prerequisites are met. Always run /validate first.
Steps (High level):

• Backup, suspend BitLocker.
• Boot WinRE or run as admin with /allowFullOS where appropriate.
• mbr2gpt /validate /disk:0
• If validation succeeds: mbr2gpt /convert /disk:0
• Reboot to firmware, set Boot Mode to UEFI and choose Windows Boot Manager.

If mbr2gpt fails validation (common reasons: no free contiguous space at disk end, unusual partition layout), the non‑destructive alternatives are third‑party converters (MiniTool, AOMEI) or using gdisk (GPT fdisk) to write GPT headers followed by creating an ESP and running bcdboot — workflows documented across community recovery threads and practical guides. Always confirm disk numbers before writing.

---

Secure Boot and platform security — why EFI matters beyond boot​

Secure Boot ensures that only cryptographically signed boot components load before Windows, which substantially raises the cost and complexity of boot‑level malware, rootkits, and game anti‑cheat avoidance. When Secure Boot is combined with TPM‑based attestation, publishers and security teams can enforce stronger platform guarantees, which is why some software (notably anti‑cheat systems and recent AAA games) now require UEFI+Secure Boot+TPM. This is a major reason to convert older systems from legacy BIOS to UEFI rather than trying to patch legacy chains.
A notable operational detail: Secure Boot keys and firmware updates are maintained by OEMs. Changes in signing keys (for example, Microsoft’s key lifecycle) can affect how third‑party OSes interact with Secure Boot — a reminder that firmware and platform security are evolving ecosystems that require vendor firmware updates and careful management.

---

Practical checklist: safely inspect, repair, and avoid accidental deletion​

• Backup: Create a full disk image and copy critical files off the system.
• Confirm boot mode: msinfo32 → BIOS Mode; Disk Management → Partition style.
• If repairing, boot WinRE or a Windows install USB (Repair → Command Prompt).
• Use diskpart only for identification or to assign a temporary drive letter. Mistaking disk numbers destroys data.
• When rebuilding boot files, use bcdboot C:\Windows /s S: /f UEFI after assigning S: to the ESP.
• If converting from MBR→GPT for UEFI boot, run mbr2gpt /validate first; if validation fails, don’t force conversion — fix layout issues or use non‑destructive tools.

---

Common pitfalls and troubleshooting notes​

• "Access is denied" running bootrec /fixboot: On UEFI/GPT systems this is common; the recommended remedy is to mount the ESP and run bcdboot to rebuild the UEFI files instead of relying on legacy bootrec behavior.
• No free space at end of disk for mbr2gpt: mbr2gpt needs room for GPT headers and sometimes for a new ESP. Shrinking the OS partition by a small amount or re‑restoring an image with intentionally left unallocated space is a tested workaround. Alternative: use gdisk or a reliable GUI converter.
• BitLocker triggers recovery after firmware/partition changes: Always suspend BitLocker before converting partitions or changing firmware mode; recreate protectors after conversion.
• EFI partition too small for Windows feature updates: Some feature updates may fail on tiny ESPs (100 MB). Expanding the ESP to 300–500 MB is a practical fix. Microsoft community discussions and admins frequently recommend enlarging the ESP to avoid update failures.

---

Conclusion​

EFI/UEFI is the firmware bridge that modern Windows relies on for secure, fast, and capable booting. The EFI System Partition is small but critical: it stores the UEFI bootloaders and Boot Configuration Data that firmware reads to start Windows. Regular checks (msinfo32, Disk Management) let you confirm your system is running UEFI; when problems occur, Microsoft’s supported tools (bcdboot, mbr2gpt) and careful diskpart usage provide safe, repeatable recovery methods. For system builders and enthusiasts, moving from legacy BIOS/MBR to UEFI/GPT is best practice for modern security (Secure Boot + TPM) and storage compatibility — but it should be done carefully: back up first, validate with tools, and follow tested commands rather than improvising.
If the EFI partition is missing or damaged, rebuilding the ESP and reinstalling UEFI boot files with bcdboot is the standard, supported recovery path; converting MBR to GPT with mbr2gpt is Microsoft’s supported way to move to UEFI when prerequisites are met. Community recovery guides provide additional step‑by‑step examples and troubleshooting tips for cases where the automated tools fail.
For system administrators and power users: treat the ESP as a protected system resource, keep regular disk images, and use firmware/OS upgrades to stay current with vendor updates for Secure Boot and TPM support so your platform remains compatible with modern security requirements.

---

Source: Windows Report What Is EFI in Windows and How It Works