The virtual machine may fail to boot from an ISO because the boot order prioritizes the virtual hard disk over the optical drive, resulting in a “No bootable medium ...

I don't think Virtualbox has a UEFI option, so I think virtualbox is (rightly) booting legacy. But still it can't boot from that ISO I just tried VMWare Player I notice that one can choose UEFI or not ...

Out of curiosity I thought I'd check if the distro already on another drive (an HDD) runs. Hooked it up, turned on the laptop and pressed Esc to open the boot menu to make certain I'd be booting from ...