Yesterday I wanted to install Linux Mint on my mid 2014 MBP to play around with. I was able to install it but it would not boot after the install. Then I booted into El Cap and installed the rEFInd boot manager. It told me that installation was sucessful but when I reboot, it boots straight into El Cap. rEFInd does not show. I never had this problem when I was still running Yosemite. Does anyone know what has changed in El Cap?