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Switching between UEFI and legacy BIOS is not just a cosmetic firmware change. The two modes start Windows through different boot paths and are normally paired with different disk layouts: UEFI commonly boots from a GPT disk and an EFI System Partition, while legacy BIOS (often exposed through CSM) commonly boots from an MBR disk and an active partition. If you change the firmware mode without matching the disk and boot files, an otherwise healthy Windows installation can stop booting.
UEFI can enable Secure Boot, but UEFI by itself does not activate Secure Boot or validate anything. Before changing a setting, identify the current boot mode and partition style, back up your files, save any disk-encryption recovery key, and follow the exact instructions for your computer or motherboard.
The short answer: what changes
| Question | UEFI boot | Legacy BIOS/CSM boot |
|---|---|---|
| How firmware starts Windows | Loads an EFI boot program, normally from an EFI System Partition (ESP). | Uses a BIOS-compatible path, traditionally loading boot code from an MBR disk’s active partition. |
| Typical Windows system-disk pairing | GPT, especially when CSM is disabled. | MBR. |
| Secure Boot | Available when the firmware and operating system support it and it is configured. | Unavailable while booting through the legacy path. |
| Older hardware and software | May require CSM for devices that have no UEFI option. | Designed for older devices and software that lack UEFI support. |
| Effect of changing an installed system | Needs UEFI-compatible boot files and a compatible disk configuration. | An installation made in this mode can fail to start if the firmware is changed to UEFI without migration. |
This describes common Microsoft Windows configurations, not every operating system or firmware implementation. A machine with CSM enabled can offer both paths, and firmware menus use different names for the same concepts.
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UEFI loads an EFI program
In UEFI mode, firmware looks for a boot program in an EFI System Partition. The partition contains files in a format the firmware understands, and the firmware can maintain one or more boot entries pointing to them. The firmware does not execute the traditional legacy boot code stored in the first sector of an MBR disk.
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Legacy BIOS uses an active MBR path
In the legacy path, firmware-compatible code in the MBR transfers control to boot code on the active partition. On modern systems this behavior is commonly provided by the Compatibility Support Module. CSM is a compatibility layer, not a third boot architecture: it makes a UEFI machine behave like a BIOS machine for selected devices or operating-system installations.
Why the installation mode matters
Windows Setup writes boot files for the mode in which the installer was started. If the installation USB was booted as UEFI, Setup normally expects a UEFI/GPT arrangement. If it was booted as a BIOS or legacy entry, Setup normally creates an MBR-style arrangement. The installed copy of Windows generally continues to boot only in the mode used for its installation unless you convert or reinstall it.
UEFI, GPT, MBR and CSM: keep the terms separate
UEFI and legacy BIOS describe firmware boot behavior
UEFI and legacy BIOS answer the question, “How does firmware hand control to an operating system?” A computer can contain UEFI firmware even when Windows is currently starting through CSM’s legacy path. Seeing “BIOS” in a setup screen therefore does not prove that Windows is using legacy boot.
GPT and MBR describe disk partitioning
GPT and MBR are partition-table formats. Microsoft documents UEFI without CSM with GPT and the legacy BIOS path with MBR. CSM can permit a legacy MBR boot on a UEFI-capable machine, but the firmware setting alone does not convert a disk or rewrite Windows boot files.
GPT’s structural advantages
The UEFI specification describes GPT as providing 64-bit logical block addressing, room for many partitions rather than the four-primary-partition limit associated with traditional MBR, a primary and backup partition table, version and size fields, CRC32 integrity fields, and GUID-based partition identities. These are layout and resilience characteristics; they are not a promise that a PC will boot faster after conversion.
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What UEFI changes for security
Secure Boot is a separate control
Secure Boot checks signatures and hashes for boot-time components against keys and policies stored in firmware. It is a UEFI feature, and Microsoft guidance for a Secure Boot configuration requires UEFI with CSM disabled. Simply selecting “UEFI” does not turn Secure Boot on.
The NSA’s Boot Security Modes and Recommendations makes the distinction explicit: “no validation or protection of the boot process is granted simply by choosing UEFI over legacy mode.” You must confirm that the firmware supports Secure Boot, switch to the appropriate UEFI-only configuration, and enable Secure Boot separately. The exact labels and key-management options vary by manufacturer.
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Secure Boot certificate changes in 2026
Microsoft Support states that Secure Boot certificates originally issued in 2011 begin expiring in June 2026 and that supported Windows versions are scheduled to receive updates automatically. Because applicability depends on the Windows release and device firmware, check Microsoft’s current support guidance and your manufacturer’s notices rather than assuming that every PC has received the update. Certificate servicing does not itself prove that Secure Boot is enabled.
Check your current state before changing anything
Find the Windows boot mode
- Press Windows key + R, enter
msinfo32, and press Enter. - In System Information, locate BIOS Mode. It normally reports UEFI or Legacy.
- Record the result before entering firmware setup. A UEFI-capable motherboard can still report Legacy when CSM was used to start Windows.
Find the system disk’s partition style
- Right-click the Start button and open Disk Management.
- Right-click the disk containing the Windows partition, not an individual volume, and choose Properties.
- On the Volumes tab, read Partition style, which will normally be GUID Partition Table (GPT) or Master Boot Record (MBR).
Also note whether an EFI System Partition is present, whether the disk is encrypted, and whether you have a current backup. Menu wording can differ between Windows releases.
Read the computer’s firmware documentation
Look up the exact computer or motherboard model. The manual may call the controls Boot Mode, UEFI/Legacy Boot, CSM, Launch CSM, Legacy Option ROM, or similar. It may also distinguish between storage, network, and graphics option ROMs. A model-specific manual is more reliable than a generic instruction to “disable legacy mode.”
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How to move an existing Windows installation from Legacy/MBR to UEFI/GPT
Do not begin by flipping the firmware switch. The safe order is to prepare Windows and the disk first, then change firmware after the new boot files exist.
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Microsoft documents MBR2GPT as a possible conversion tool for supported Windows installations. Eligibility depends on the disk’s partition arrangement, Windows version, recovery configuration, and firmware. A conversion can fail validation, and a successful conversion still requires the firmware change afterward.
- Create a tested backup of personal files and, if possible, a full system image. Keep a Windows recovery drive or installation medium available.
- If BitLocker or another disk-encryption product is active, confirm the recovery key is available and follow the product’s instructions for suspending protection during firmware changes. Microsoft warns that changing firmware configuration can trigger recovery or cause startup failure when encryption is not handled correctly.
- Open an elevated Command Prompt and run the validation phase recommended for your Windows release, such as
mbr2gpt /validate /allowFullOS. Do not proceed if validation reports a problem; correct that problem or use a clean installation instead. - After validation succeeds and you have verified your backup, run the conversion command documented for your release, commonly
mbr2gpt /convert /allowFullOS. The tool creates an EFI System Partition and installs UEFI boot files while retaining the Windows installation when the disk meets its requirements. - Restart into firmware setup. Change the boot mode to UEFI, disable CSM or legacy boot as the manual directs, and select the new Windows Boot Manager entry if one appears. Do not enable unrelated option-ROM settings unless the manual requires them.
- Allow Windows to start before making further changes. Recheck System Information to confirm that BIOS Mode now reports UEFI and verify that your data and applications are intact.
Those commands are examples of the Microsoft tool’s normal workflow, not a guarantee that every installation qualifies. Use the current Microsoft procedure for your Windows version and the machine maker’s instructions.
Option 2: perform a clean UEFI installation
A clean installation is appropriate when the existing layout is unsupported, the system is being rebuilt, or you do not need to preserve the current Windows installation. It is also the most predictable way to obtain a fresh GPT/UEFI setup, but it can erase the disk.
- Back up documents, application data, browser profiles, license information, and anything else you need. Confirm that you can reinstall required applications.
- Create Windows installation media and enter the firmware boot menu. The same USB may appear twice, for example as a UEFI entry and a non-UEFI or BIOS entry.
- Select the UEFI-labeled entry. Starting the USB in the legacy entry can make Setup expect an MBR installation instead.
- At the disk-selection screen, delete or reformat partitions only when you have deliberately chosen to erase that disk. Setup can then create the required GPT partitions automatically.
- After installation, verify the boot mode in Windows and apply device drivers, updates, encryption recovery settings, and backups before restoring data.
What happens if you switch UEFI to Legacy
This direction is less common on current Windows PCs, but it may be needed for an older operating system, an old expansion card, or software that has no UEFI support. A Windows installation created as UEFI/GPT normally has no legacy MBR boot path. Switching to Legacy or enabling CSM without converting or reinstalling can therefore produce a “no boot device” message.
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- Confirm that the target operating system supports legacy boot and that the firmware still provides CSM.
- Check whether the disk is GPT. A legacy installation normally needs an MBR boot arrangement; changing only the firmware mode does not create one.
- Disable Secure Boot before attempting a legacy boot, because Secure Boot applies to the UEFI path and is not available through legacy boot.
- Use a compatible installation or recovery medium and follow the manufacturer’s exact CSM and option-ROM settings.
- Remember that some modern Windows requirements assume UEFI and Secure Boot; moving to legacy mode can make those requirements impossible to satisfy.
Choosing the correct mode for an installation USB
Boot menus often list the same removable drive twice. Choose the entry that matches the installation you intend to create:
| Intended result | Boot-menu choice | Likely Setup expectation |
|---|---|---|
| Modern UEFI installation | Entry explicitly prefixed with UEFI | GPT disk and an EFI System Partition |
| Legacy-compatible installation | Non-UEFI or BIOS entry, when offered | MBR disk and an active partition |
Do not alternate between entries while troubleshooting without recording what you selected. Booting the same USB in different modes can lead Setup to create different partition layouts and can leave you with an installation that only starts in one mode.
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“No boot device,” “Operating system not found,” or a firmware screen
The usual cause is a mismatch: the firmware is now looking for UEFI files while Windows has only a legacy MBR boot path, or it is looking for legacy code while the installation has only an EFI System Partition. Return temporarily to the mode that previously booted Windows. If that restores startup, back up your data and perform a planned conversion or reinstall rather than repeatedly toggling the setting.
Windows Boot Manager is missing from the boot list
In UEFI mode, the firmware should normally show a Windows Boot Manager entry associated with the EFI System Partition. If it does not, confirm that CSM is disabled as required, check that the disk is detected, and use Windows recovery media to repair the UEFI boot files. A recovery environment cannot make an MBR-only installation boot in UEFI without the appropriate conversion.
BitLocker asks for a recovery key
A firmware or boot-chain change can alter the measurements used by disk encryption. Enter the saved recovery key only after confirming that you made the intended change. If you cannot produce the key, stop changing settings and use the organization’s or manufacturer’s recovery process; do not assume that reinstalling will preserve encrypted data.
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Setup refuses to install because of partition style
This normally means the USB was started in one mode while the disk is prepared for the other. Cancel before deleting anything, reboot the USB through the desired UEFI or legacy entry, and then choose the appropriate conversion or clean-install path. A clean-install conversion or disk cleaning is destructive unless all needed data has been backed up.
Compatibility, performance and trade-offs
Older hardware
Legacy mode remains useful for old graphics cards, storage controllers, network adapters, and operating systems whose option ROMs do not provide UEFI drivers. UEFI-only mode can prevent those devices from initializing. CSM is a compromise when the motherboard supports it, but enabling CSM may disable or limit Secure Boot and may change which boot entries are visible.
Modern Windows requirements
UEFI with GPT is the normal foundation for current Windows deployments, particularly where Secure Boot is required. The exact requirement depends on the Windows release, hardware and organizational policy; do not infer compliance from the word “UEFI” alone. Verify both the firmware mode and Secure Boot status.
Boot speed
UEFI can support faster initialization on some hardware, but the partition-table format itself does not guarantee a measurable boot-time improvement. Device initialization, firmware settings, storage, drivers and startup software often dominate the result. No universal performance figure applies to every machine.
A practical decision guide
| Your situation | Safest direction | Why |
|---|---|---|
| Windows reports Legacy and the disk is MBR; you want Secure Boot or a modern UEFI setup. | Back up, validate an eligible MBR2GPT conversion, convert, then switch firmware to UEFI. | It preserves the installation when supported and aligns the disk and boot files before the firmware change. |
| Windows reports Legacy, conversion validation fails, or the installation is damaged. | Back up and perform a clean installation started from the USB’s UEFI entry. | Setup can create a new GPT/UEFI layout without relying on an unsupported conversion. |
| Windows reports UEFI and the disk is GPT; you need an old device or operating system. | Keep UEFI unless the device genuinely requires CSM; test the legacy operating system on separate media or a separate disk first. | Switching the existing installation to legacy can make it unbootable and removes Secure Boot. |
| You only want Secure Boot. | Remain in UEFI, disable CSM if required, and enable Secure Boot as a separate step. | UEFI is a prerequisite for the feature, not the feature itself. |
Bottom line
UEFI and legacy BIOS are different boot paths, and the path is tied to how Windows’ boot files and the disk’s partition table were created. UEFI normally goes with GPT and an EFI System Partition; legacy BIOS/CSM normally goes with MBR and an active partition. Secure Boot is available only through a supported, configured UEFI setup and must be enabled separately. Identify your current mode and disk style, back up your data and encryption keys, then convert or reinstall before changing firmware. Flipping the setting first is the fastest way to turn a working installation into a temporarily unbootable one.
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