A “UEFI-bootable” USB flash drive and can I boot it on legacy BIOS


I've learned the difference between UEFI and BIOS, and it seems that BIOS only boots MBR hard drive while UEFI only boots GPT hard drive. And UEFI cannot boot normal USB flash drive so I have to make the USB flash drive "UEFI-bootable". What is the difference between normal USB drive and "UEFI-bootable" drive? Is it true that a normal USB drive uses MBR while a UEFI-bootable one uses GPT? Can I boot a UEFI-bootable USB flash drive on an old PC with only legacy BIOS support?

Best Answer

Most UEFI implementations can also boot from MBR-partitioned USB drives. They expect a file at /efi/boot/bootx64.efi to exist. For best compatibility, using the FAT32 filesystem is highly recommended.

An additional BIOS bootcode may exist on the drive. It will be ignored by UEFI.

Bottom line: Hybrid bootable USB drives are possible and do exist.