I made a Linux Ubuntu 16.04 VDI image on VirtualBox on an Intel i7 machine (4 CPUs), with a set of tools inside. Then I copied that VDI to an old machine (Core 2 duo - 2 CPUs), I created the VM using the VDI file copy. The image boots, but runs very slowly with 1 CPU (which is obvious).
The problem now is that when I use all the 2 cores (in the older machine) the virtual machine doesn't boot, it displays the GRUB page, then a black screen (with a blinking cursor _ ). Yet, the boot succeeds in the i7 machine with any number of cores (1-4). Is there something I missed?