Make USB drive with Ubuntu Linux recognized at startup or startup manager

I have created a .dmg file and followed the instructions on the ubuntu website, but the usb drive is not on the list of items to startup or in the startup manager. Why? What do I need to do?

mac 10.开发者_开发知识库4.11

sudo dd if=ubuntu.dmg of=/dev/rdisk1 bs=1m

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http://www.makeuseof.com/tag/how-to-create-an-ubuntu-installation-usb-on-the-mac/

a nice tutorial

maybe you want to install refit as well (as a bootloader)