The UEFI firmware has replaced the traditional BIOS on almost all HP PCs manufactured in recent years. Switching to Legacy mode (also known as CSM) is sometimes necessary to boot an older operating system or a cloning utility that is incompatible with UEFI. This action changes how the motherboard initializes the hardware and searches for a bootable system on the disk.
GPT or MBR Partition Table: What Changes Before Touching the HP BIOS
Before opening any BIOS menu, it’s important to understand the link between the boot mode and the disk format. A system installed in UEFI uses a GPT partition table. A system installed in Legacy relies on an MBR table.
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Changing the boot mode without adapting the disk makes Windows unbootable. Several reports from HP ProBook and EliteBook users confirm that a simple switch from UEFI to Legacy in the BIOS breaks the boot of the existing installation, even if one later switches back to UEFI. The disk must then be re-prepared and the boot entries rebuilt.
To check the current disk format in Windows, open the Disk Management tool (right-click on the Start menu), then right-click on the system disk and check the Volumes tab. The “Partition Style” line indicates GPT or MBR. Knowing how to read this information avoids switching an HP from UEFI mode to Legacy mode without understanding the consequences on booting.
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Accessing the HP BIOS and Enabling Legacy Boot Mode
On most HP laptops and desktops, the key to access the BIOS is F10, pressed as soon as the HP logo appears during startup. On some models, Esc first opens an intermediate boot menu where you then select “BIOS Setup”.
Navigation in the Boot Menu
Once in the BIOS, go to the tab often named “System Configuration” or “Boot Options”. Two settings need to be changed in this order:
- Secure Boot must be disabled first, as it is incompatible with Legacy mode. This option is sometimes found in a dedicated “Security” submenu.
- The “Legacy Support” or “CSM Support” option must then be enabled. On HP BIOS, it is often grayed out as long as Secure Boot remains active.
- The boot order must be rearranged to place the desired Legacy device (hard drive, USB drive) first.
Confirm with F10 (Save and Exit). The PC will restart. On several HP models, a confirmation screen appears with a code to type on the keyboard to validate the change of Secure Boot. This security step prevents accidental modification.
Recent HP BIOS Cases Without Legacy Option
In recent years, many HP laptops no longer offer Legacy boot. Recent BIOS versions shipped with Windows 11 only provide UEFI Native and UEFI Hybrid modes. Downgrading the BIOS version does not restore the CSM option on these machines. If the Boot Options tab contains no mention of Legacy or CSM, the hardware simply does not support it.
Converting MBR Disk to GPT Without Windows Reinstallation
The reverse scenario is common: after using Legacy mode, switching back to UEFI requires converting the disk from MBR to GPT. Microsoft has provided the mbr2gpt.exe tool since Windows 10 version 1703. This command-line utility converts the system disk without data loss or reinstallation.
The command is executed from a Windows PE environment or directly from Windows with the /allowFullOS option:
- Open a command prompt as an administrator.
- Type mbr2gpt /validate /disk:0 to check that the disk is eligible for conversion (no more than three primary partitions).
- If validation succeeds, run mbr2gpt /convert /disk:0 to perform the conversion.
- Restart, access the HP BIOS (F10), re-enable UEFI mode and Secure Boot, then adjust the boot order.
The conversion fails if the disk contains more than three primary partitions or if an extended partition is present. In this case, a full backup followed by a clean reinstallation in UEFI/GPT remains the reliable method.
Consequences of Switching to Legacy on an HP PC Running Windows 11
Disabling UEFI and Secure Boot on an HP shipped with Windows 11 has effects beyond just changing the boot screen. Secure Boot is a software prerequisite for certain games and anti-cheat: Riot Vanguard, for example, requires UEFI with Secure Boot active. Switching back to Legacy cuts access to these software even if Windows still functions.
BitLocker encryption, enabled by default on many HP business machines, relies on TPM and Secure Boot. Changing the boot mode triggers the request for the BitLocker recovery key upon restart. Without this key (stored in the Microsoft account or in Active Directory), the disk becomes inaccessible.
Switching to Legacy remains relevant for specific cases: booting an old diagnostic utility from a USB drive, installing an operating system that does not manage UEFI, or bypassing a specific hardware compatibility issue. Outside of these situations, keeping UEFI mode with Secure Boot active better protects the boot chain and maintains compatibility with modern tools.