Persistent Ubuntu boot USB: macOS Sierra
This following method installed a persistent boot USB with Ubuntu 16.04.2 LTS on my mid 2012 2GHz i7 MacBook Air running macOS Sierra:

They offer it for like $5US, but I am a cheap S.O.B., so from their GitHub repo...


Download Mac Linux USB Loader from the good folks at SevenBits.
Install Cocoapods (gem install cocoapods).
cd into the GitHub directory and pod install.
Launch "Mac Linux USB Loader.xcworkspace" in XCode (require XCode 8+ and registration as a Developer) and run the app. in the sandbox.
Insert and mount a USB drive &amp; use Disk Utility to Erase to MS-DOS(FAT) and GUID Partition Map Master Boot Record.
In the "Persistence Manager" set the allocation of storage for the persistent memory (this creates the casper-rw file).



(source: sevenbits.io)  


In the "Create Live USB" menu, point the "Create Live USB" to the Ubuntu ISO...





Deselect the "Skip the boot selection menu" option and let 'er rip!
Once finished, restart and hold the alt/option button, then select the EFI boot disk. Note the boot procedure here: https://github.com/SevenBits/Mac-Linux-USB-Loader/wiki/Using-Persistence-with-Mac-Linux-USB-Loader and after the drive is selected, boot to persistent by selecting from the menu options: 2, then 1, then 0.
Of note: once booted into Ubuntu, I created an admin user account, changed the desktop, then ran touch test.txt and sudo apt install valgrind to see if the changes would stick. When I logged out and logged back in, the b.g. reset to default, but the user account, text.txt file and valgrind install remained. When I rebooted, the changed desktop picture had stuck... A little funky, and it might not be a "dual boot OSX &amp; Linux" lappy, but so far it's persistent Ubuntu on a stick!




NOTE: I reconfigured the persistent USB by formatting the USB using Master Boot Record instead of GUID. It feels like the startup is snappier. Both USB drive formats result in the console displaying the the command "applesetos" is not found. Not sure what this means. Per the maker, "The applesetos message is an oversight from me, but won’t affect the boot process." Similarly, with Master Boot Record I had to first create an admin account, and restart before system changes started taking hold.