This method works with *unix ISO files. If you want to create a bootable Windows USB, use Bootcamp Assistant instead.

OS X shipped with dd installed. It’s a little utility to copy/convert from an input (your ISO file) to another output (your USB stick). The process is fairly easy.

Plugin your USB stick. Find where your USB stick is mounted by looking at the name and the size. It looks like /dev/diskX.

diskutil list

To burn the bootable ISO to USB stick, issue the following command. Replace the ISO file name with the path to your ISO file and diskX with the value you noted at the previous step.

sudo dd if=xubuntu-14.10-desktop-amd64.iso of=/dev/diskX

Enter your password, wait for a little while. It may looks like it’s hanging but do not abort. It would take few minutes, depends on the size of the ISO file.

If you find dd too slow, you can try replace /dev/diskX with /dev/rdiskX and add bs=4m option.

sudo dd if=xubuntu-14.10-desktop-amd64.iso of=/dev/rdiskX bs=4m