Dual booting Mid 2012 Macbook Pro multiple SSDs Errors 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. 

Here is a photo of my current Disk Utility config.

Any advice?

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.
If you can't confirm your hard drive and don‘t want to overwrite you OS X drive.
You can unplugged one driver to test boot into macOS,leave only one empty hard drive on the Mac then start install.
Use option to enter the boot manager.
And you can choose 'Erase disk and install Ubuntu'.