How to correctly copy a virtual machine in VirtualBox

Export

1. File
2. Export Appliance
3. Follow the assistant to save a .ova file

Import

1. File
2. Import Appliance
3. Follow the assistant to import the virtual machine (select the .ova file)

In case of error when starting the machine, READ THE ERROR MESSAGE. Most of the time it also gives a quick fix, so read it before asking... really. If you get any error READ IT.