The first partition should be as small as possible but not less than 200MB. Next, partition the drive such that you have two partitions, and the first should have the 'OS X Extended (Journaled)' filesystem. The purpose of this is to ensure that your hard drive uses the GPT Partition Map and therefore bootable from EFI. Reformat the hard drive so that the Format is 'OS X Extended (Journaled)' and the Scheme is 'GPT Partition Map'. Instead of selecting your internal hard drive, select the drive you want to install Ubuntu to. Under certain circumstances you can do it without wiping, but I won't cover that here. Instead of 50 GB free space, you need instead a hard drive that you don't mind wiping. I have followed this (, mirrored on: ) guide with the following modifications. You should be able to try this on your system without fear of irreversibly altering your primary hard drive and system. My directions are for installing Ubuntu 16.04 LTS on a 2014 MacbookPro 13-inch Retina model for a system with EFI after installation, you should be able to boot an Ubuntu installation on an external drive from your Mac's native EFI software. The problem with naively installing Ubuntu on an external drive is that your Mac might not recognize it as bootable, due to its custom implementation of the EFI and UEFI specs.

Yes, in fact I have just done it yesterday!

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