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The UEFI specification requires MBR partition tables to be fully supported.

UEFI firmware does not execute the code in the MBR, except when booting in legacy BIOS mode through the Compatibility Support Module (CSM). On legacy BIOS-based systems, the first sector of a partition is loaded into memory, and execution is transferred to this code. UEFI provides backward compatibility with legacy systems by reserving the first block (sector) of the partition for compatibility code, effectively creating a legacy boot sector. Also, El Torito bootable format for CD-ROMs and DVDs is supported. Both GPT- and MBR-partitioned disks can contain an EFI system partition, as UEFI firmware is required to support both partitioning schemes. The globally unique identifier (GUID) for the EFI system partition in the GUID Partition Table (GPT) scheme is C12A7328-F81F-11D2-BA4B-00A0C93EC93B, while its ID in the master boot record (MBR) partition-table scheme is 0圎F. The actual extent of divergence is unknown: Apple maintains a separate tool that should be used, while other systems use FAT utilities just fine.

The EFI system partition is formatted with a file system whose specification is based on the FAT file system and maintained as part of the UEFI specification therefore, the file system specification is independent from the original FAT specification. See also: UEFI § UEFI booting, and UEFI § CSM booting