How can I create a bootable USB drive for Ubuntu from OS X?
I have tested what follows on two different MacBook Pros:


13-inch, Mid 2010 (2.66 GHz Intel Core 2 Duo, 8GB RAM, Nvidia GeForce 320M 256MB)
13-inch, Late 2011 (2.8 GHz Intel Core i7, 16GB RAM, Intel HD Graphics 3000 512MB)


Overview

Create Bootable USB Drive using Startup Disk Creator in Ubuntu itself.
Note: I use Ubuntu daily as a Tomcat Web Server and Java Development environment, and I do so in a Virtual Machine. I understand this is not the most ideal arrangement for someone wishing to create a bootable USB disk to boot a Mac to, but it works, and has proven reliable for me. If you are bent on using Ubuntu natively and want to test it first, a VM is one way to do it, and booting to USB is another. For the full-on experience, installing Ubuntu to your hard-drive (after taking the appropriate measures to back up your harddrive, etc... &lt;disclaimer&gt;I take no responsibility for lost data, etc... &lt;/disclaimer&gt;)
There are minor nuances to each of these scenarios:


VM leverages the host OS Networking, so there are no issues with Broadcom drivers
VM will have issues with x/y coordinates of the display (0,0 being the upper left hand corner, for example), meaning hot-corners in Ubuntu will not work properly (for things like auto-hiding the launcher bar)
USB Boot will support your graphics card (most likely) and x/y coordinates will be mapped properly.
USB Boot will not have native WiFi support, so you will have to download Broadcom drivers using a wired connection


This is true whether or not you are booting from USB or have Ubuntu installed onto your hard drive



Instructions



I use VMWare Fusion to create my VM's, which is a paid application, though there is a free trial. However, VirtualBox is free to download and use and from what I understand, works quite well also.

That said, I created a Bootable USB Stick using Startup Disk Creator in my Ubuntu 13.10 VM, so I will explain here the steps:


Install Ubuntu 13.10 in Virtual Machine.


If this does not quell your curiosity as to whether or not to run Ubuntu natively, then proceed...

Download the Ubuntu 13.10 Mac image from here


NOTE: There are 3 links to Ubuntu 13.10 for Desktop. You want to make sure to download the last one... labeled "64-bit Mac (AMD64) desktop image"

Insert a blank USB Key into the Mac, and if given a choice of which system to connect it to (VMWare Fusion recognizes this, and asks what to connect it to, Mac or your VM) choose the VM.


This should mount the USB in the VM as a drive (Eg., it should have a mount point in /media)


Open Startup Disk Creator in Ubuntu and choose the target device as the USB drive, and the source as the Ubuntu 13.10 ISO image you downloaded.


Accept the defaults, and click on "Make Startup Disk"
Close out all open applications (including VMs)
Shutdown your Mac.
Insert USB drive into Mac if it is not already
Press Power button to turn on the Mac, and immediately press and hold the Option key.
You should be presented with 2 or 3 boot options.


Mac OS Startup Disk
Mac OS Repair Partition *may not be present
EFI Boot (this should have an Orange USB Drive icon)

Choose the EFI Boot.


NOTE: If you use an Mac with NVidia graphics, you will see a scrambled Boot animation (The Ubuntu logo with the pulsing orange dots). This is due to the fact that Ubuntu defaults to the Open Source drivers for NVidia hardware, and they are not loaded yet. If you choose to install natively, you may want to try the proprietary binary drivers provided by NVidia.

The USB drive you see referenced in that screen shot has Ubuntu 13.10 on it and Boots successfully on the two systems mentioned above.