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I'm using a Mac computer and I'm trying to install Ubuntu on an external HDD. I have the iso file for installing the OS but after following these instructions from Ubuntu to create a bootable USB stick, I have an error saying that the USB stick can't be recognised and I have 3 options : initialise, ignore and eject. When I click initialise Disk Utility opens. When I try to boot up from the USB, the flash drive isn't listed as a bootable device. What should I do?