Virtual Box and external hard drive running Mac OS

[YasRebl]

I just want to confirm what I've gathered from searching on the forums. VirtualBox cannot be used to load/boot MacOS already installed and running on an external hard drive?

As I'm running Mojave, I was hoping to run Yosemite at the same time using a virtual machine. Somewhere online was written that one could achieve this by using VirtualBox, but so far what I've seen is, VirtualBox is used to create a virtual machine AND install a different OS to that VM, not that I can use it to boot an OS already installed on an SSD.

Thank you in advance to anyone who comments to this thread.

[socratis]

VirtualBox cannot be used to load/boot MacOS already installed and running on an external hard drive?

Of course you can! I'm on 10.11.6 and have a 10.13 beta (yes I know it's old) running from an external HD. I need to upgrade it to 10.14 at some point...

What you do is to install OSX (pick your version) to the external HD, normally by running the installer from your current installation. I'm not sure if a newer OSX version will let you install an older OSX version on an external HD, never tried it. You might have to create an ISO from the installer.app, create a bootable DVD and boot from that in order to install it. And even then, it might not go through if your computer is not compatible.

You don't have that problem, because you already have OSX installed on the ext. HD. I simply mention it for completion...

After that, you create a VMDK file that points to the whole HD, and you're done. For the second part, there's even a utility that makes it piece of cake if you don't want to mess with the command line: VBoxRawdisk - a macOS utility to generate rawdisk VMDKs.

Moving to "OSX Guests" from "Using VirtualBox".