After installing onto a GPT labeled disk that had previously used MBR partitioning, sometimes the x86 firmware can not boot from the disk. This happens if your BIOS implementation requires that at least one hard disk have at least one MBR partition that is marked as bootable to boot in BIOS mode. The problem exists because UEFI specification forbids the EFI protective MBR partition from being marked as active. To fix this situation, use the fdisk command to set the partition's status to be active before attempting to boot.