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.

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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