Then choose erase to delete all the data from the virtual disk.
Then you will need to uncheck the option for “Enable EFI (special OSes only)” as on the screenshot below.
You can freely install any OS on your machine without any interruption from your current OS. VirtualBox is a world-famous application that offers its users plenty of advanced tools for effective customization.
VIRTUALBOX SIERRA CODE HOW TO
How to install Snow Leopard on VirtualBox 4