An Hybrid UEFI GPT + BIOS GPT/MBR boot USB disk is a disk that works on both UEFI and BIOS PCs.
Requisites:
A working Linux installation with the following packages: grub2, grub2-efi, gdisk (on Debian Jessie: grub-pc-bin, grub-efi, grub-efi-amd64, gdisk) An USB drive Some Linux ISO.