- Download your preferred Ubuntu ISO
- Create a new linux virtualbox with a new HD
- Attach the Ubuntu ISO to the new virtualbox
- Install Ubuntu to the virtualbox
- Shut down the virtualbox
- Temporarily detach the virtualbox’s .vdi hard drive image
- Make the .vdi image “immutable”:
VBoxManage modifyhd YOUR_IMAGE.vdi --type immutable
- Reattach the .vdi image to the virtualbox