Hello everyone, while I have seen similar posts in regards to installing a different Mac OS on an external hard drive, I haven't seen anywhere the specifics to what I'm trying to do and hopefully I can get the help in this forum. I am currently running El Capitan (10.11.6) OS in my Macbook Pro mid 2015. I just got an external SSD which I partitioned because I want to install and run Yosemite (10.10) OS on it because of some software compatibility issues I have which I need to run on Yosemite. Obviously, if I try to open the Yosemite installer I get the This copy of the “Install OS X” application is too old to be opened on this version of OS X message. As an alternative, I tried creating a bootable USB drive installer of the Yosemite OS, which can be seen from the System Preferences > Startup Disk menu as well as when I restart the Macbook holding Option (Alt). The issue I am having is, when I select the installer USB drive from this screen, I get a black screen with the NO ENTRY sign. After searching all over the internet (youtube, google, tutorials, forums, etc), I haven't been able to find a way to do what I'm trying to do. So, do any of you have any idea how to achieve this? Thank you in advance for any help or pointing me in the right direction.