I have an old Mid-2012 Macbook Pro. I replaced the the old HDD with a new SSD and removed the optical drive and replaced it with another SSD. I migrated everything from the old HDD to the new SSD without any issues.
Now I am trying to install Xubuntu on the other SSD using this guide. However this only applies to a single SSD with a few partitions. I just want to have the Mac OS X on one SSD and Xubuntu on the other.
When I boot into the installer for Xubuntu, I cannot tell my SSDs apart. Which is not a big deal I just do not want to overwrite my OS X drive. When I select one at random I am getting an error about boot files/partitions.
EDIT After some trial and error I more or less came to the same conclusion as @maP1E bluE. I simply unplugged the Mac OS X boot drive, plugged in the Xubuntu USB, and booted into the empty drive. The installation process went through without any major issues. minus the Wi-Fi NIC firmware issue. By default I boot into Xubuntu which is fine and if I need to access the Mac OS X install, I simply press and hold the option button and select the other drive.