UEFIonly

UEFIonly is a term used to describe computer systems whose firmware and boot environment operate exclusively in the Unified Extensible Firmware Interface (UEFI) mode, without the option to boot via legacy BIOS compatibility (often provided as the Compatibility Support Module, or CSM). In UEFI-only configurations, the boot process relies on the UEFI boot manager and EFI executables (.efi) stored on the disk's EFI System Partition.

Typically, disks used for UEFI boot are partitioned with the GUID Partition Table (GPT) and include an EFI System Partition that houses bootloaders such as bootmgfw.efi for Windows or grubx64.efi for Linux. Legacy MBR booting is not used; operating systems and bootloaders must provide UEFI-compatible installers and bootloaders.

Advantages include faster boot times, more flexible boot options, and the ability to enable Secure Boot, which requires signed bootloaders and kernel modules. UEFI can also support larger disk partitions and modern hardware features.

Disadvantages or considerations: Some older operating systems, peripherals, or recovery tools may lack UEFI support; dual-boot configurations with BIOS-mode OSes require CSM or separate drives, and some firmware offer only one mode or a mixture; enabling Secure Boot may complicate third-party bootloaders.

Implementation and administration: In practice, users set their firmware to UEFI-only by disabling CSM or Legacy Boot. For new installations, GPT and UEFI boot are common defaults; for existing systems, converting from MBR to GPT and enabling UEFI requires caution and backups. Some virtualization platforms also provide UEFI firmware options for guest machines.