The BIOS Boot partition is a partition on a data storage device that may be used by standard BIOS-based machines in order to boot when the partition table of the device is a GPT label.
The Globally Unique Identifier for the BIOS Boot partition in the GPT scheme is 21686148-6449-6E6F-744E-656564454649 (stored on disk as "Hah!IdontNeedEFI" if interpreted as ASCII). The formation of this "GUID" does not follow the GUID definition; such formation of "GUID" value breaks down the guaranteed uniqueness of GUID. In the context of GPT on a BIOS-based computer, a BIOS Boot partition is similar in some respects to the EFI System partition, which is used by systems based on EFI. The EFI System partition, however, holds a filesystem and files used by the EFI, whereas the BIOS Boot partition is accessed without a filesystem and holds raw binary code.
When used, it contains the second stage of the boot loader program (the first being the code that is within the MBR). Use of this partition is not the only way BIOS-based boot can be performed on GPT labels; however, complex boot loaders such as GRUB 2 cannot fit entirely within the confines of the MBR's 440 or 446 bytes of space and so need an ancillary storage space. On MBR disks, such boot loaders typically use the sectors immediately following the MBR for this storage. On GPT disks, no equivalent space exists, and the BIOS Boot partition is a way to officially allocate such space for use by the boot loader. BIOS Boot partitions are used primarily by GRUB 2.
The BIOS Boot partition is typically quite small. It can be as small as about 30 KiB; however, future boot loaders might require more space, so creating a larger BIOS Boot partition is advisable. Because of the 1 MiB partition alignment policies used by most modern disk partitioning tools to provide optimum performance with Advanced Format disks, SSD devices, and some types of RAID configurations, 1 MiB is a logical size for a BIOS Boot partition.
The following utilities are known to support BIOS Boot partitions:
A BIOS Boot partition can be created using any of several different disk utilities: