Comment installer Ubuntu sur MacOS X El Captain. C'est le créateur de Refind qui répond à la question. Testé et approuvé

1. Boot the Ubuntu installer into "try without installing mode."
2. Open a Terminal program window.
3. In the Terminal, type ubiquity -b. This will run the Ubuntu installer, but -b tells it to not install a boot loader. If you follow advanced installation options, be sure to use ext4fs as your filesystem. Do not use a separate /boot partition unless you use LVM, RAID, or an encrypted root (/) partition. You may need to shrink your OS X partition to make room for Ubuntu (or you can do that before you start).
4. When you're done, reboot, but hold down Command+R to boot into the OS X recovery environment.
5. Launch a Terminal window and type csrutil disable to disable SIP.
6. Reboot. The system should boot OS X normally.
7. Install rEFInd.