if your system has an EFI she'll and you want to boot from a shell to attached media, here is how.
1: Make sure the media with. EFI compatible OS is connected to the system.
2: Boot to a EFI shell
3: # map -r (this will remap and display all attached media)
4: change location to the drive that contains your target OS. This ca be done by typing the name and partition number of the required mount. For example; # FS0:
5: # ls to see the directory's contents
6: # cd to /EFI/boot
7: this directory will contain 1 or more boot.efi files. Run the version you need by typing its name and extension
TIP: EFI shells support tab completion