Legacy MBR boot is not able to recognize GUID Partition Table (GPT) disks. It requires an active partition and supporting BIOS to facilitate access to disk.

Does GPT work with BIOS?

Non-boot GPT disks are supported on BIOS-only systems. It is not necessary to boot from UEFI in order to utilize disks partitioned with the GPT partition scheme. Therefore you can take advantage of all of the features offered by GPT disks even though your motherboard only supports BIOS mode.

Is Legacy MBR or GPT?

In general, the legacy BIOS only supports MBR, but the UEFI supports MBR and GPT. ... All operating systems can be installed on the MBR disk. However, all Windows versions can use GPT partitioned disk to store data, but booting is only supported for 64-bit editions on UEFI-based systems.