How to create USB Ubuntu boot disk in OS X so that it boots on *non-Apple* computer?
For me it has always worked (albeit building Mint install usb pen drives DOS MBR partitioned) using Terminal , and as per the instructions on the Linux Mint website (http://community.linuxmint.com/tutorial/view/744), which I copy below:


  Using the Terminal
  
  It is really simple. Go to a Terminal and type:
  
  sudo dd if=~/Desktop/linuxmint.iso of=/dev/sdx oflag=direct  bs=1048576
  Where '~/Desktop/linuxmint.iso' is the name and location of your downloaded image (located at the desktop in this example) and '/dev/sdx' is the target USB drive. If your system doesn't support 'oflag=direct', you can just leave it out as it is simply intended to speed up the process a bit.
  
  Remember, don't include an integer for the USB drive, e.g. '/dev/sdx1', as it would refer to the existing partition on that drive and not the drive itself.
  
  When the USB has been properly created by 'dd', there should be an output similar to this:
  
  sudo dd if=~/Desktop/linuxmint.iso of=/dev/sdb oflag=direct bs=1048576
  
  706+1 records in
  
  706+1 records out
  
  740601856 bytes (741 MB) copied, 91.7024 s, 8.1 MB/s


Hope it helps