Problem with setting up a bootable USB on corrupted OSX

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http://askubuntu.com – I cant access the desktop because my harddrive is corrupted. Therefore I want to make a bootable USB stick so i can transfer all of my important files to my External HD. Problem is that Im stuck on Step 3 ---> http://www.ubuntu.com/download/help/create-a-usb-stick-on-mac-osx . Im not sure of what i should change this to: hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso It says convert failed - No such file or directory. Obviously the problem is that i dont know the path to the USB, maybe you know how to find out? Please help me. Thanks. ps. Ive downloaded the ubu (HowTos)