Ubuntu – Boot usb key on both UEFI and MBR


I installed XUbuntu on a fast usb key on a new computer using UEFI mode.
When I try to boot on this usb key with a old computer, it doesn't work.

After F12 and selected USB, the screen is black with an horizontal cursor is blinking.
Obviously, the MBR is not set up or my bios doesn't support GPT tables. (laptop from 5years ago)

Where and how to check if my usb key is able to being booted on both ways ? In the case the MBR boot is unable, how to create it beside the UEFI mode ?

The CPU is an Intel Core2Duo T9900.
The USB stick is a Sandisk Extreme 64GB. It's pretty weird because my windows 7 is a 64bits version and of course XUbuntu installed is the 64bits version.

Best Answer

BIOS and EFI require entirely different boot loaders. (To be sure, GRUB2 is available for both environments, but those are different GRUB2 binaries -- sort of like Firefox being available for both Windows and Linux.) Thus, to boot on both BIOS and EFI computers, a medium needs to include both BIOS and EFI boot loaders. You've already got an EFI-mode version of GRUB2 installed, so that means you must install a BIOS-mode boot loader. I recommend avoiding GRUB2 for this, simply to avoid the possibility of conflicting or confusing GRUB2 configuration files. That said, it is possible to get both BIOS-mode and EFI-mode GRUB2 installations to coexist; I simply don't know the details of any "gotchas." You might want to look into SYSLINUX for this task, but I'm afraid I can't provide step-by-step installation instructions.