Thanks for the link, but I'm still new to all this. Not sure I understood properly. Since I'm trying to install Catalina on an USFF Optiplex I think I could not add another disk.
Should I follow this other guide?
I have just completed an installation of OS X, Windows 10 and Ubuntu 16.0.4 LTS on a single 1Tb HDD on an Asus P8Z77i-Deluxe. It is amazing how easy it is today to Multiboot these 3 OS's on UEFI systems. Here is what you need: Here is how it goes: Create your USB installers and have them...
www.tonymacx86.com
When do I install Win10, "before running Multibeast"?