I had two machines: OSX Yosimite with VirtualBox 4.3 and a Win 7 guest OSX Yosimite with VirtualBox 4.3 and a Win 8.1 guest
Both had the Win 10 upgrade icon but they both said "Unfortunately, this PC is unable to run Windows 10.". It cites a compatability issue with the VirtualBox Graphics Adapter for Windows (both 7 and 8).
I upgraded to El Capitan and Virtual Box 5 on one and I upgraded to VirtualBox 5 on Yosimite for the other but have the same message on both.
Anybody know how I can get a Graphics Adapter for VirtualBox 5 that will let me upgrade to Windows 10?