Hey Chris,
additionally to Alexander guide I'd like to add that Virtual Disk Image (VDI) files carry a UUID for identification that is also specified in the .vbox file. So please make sure that the UUIDs of the VDIs match those in the configuration files. For vm_ubuntu_16_04_64.vbox
<HardDisks> <HardDisk uuid="{2a248ff9-7ac9-4862-98f9-14e31968f23d}" location="/ubuntu_16_04_64.vdi" format="VDI" type="Immutable"> <HardDisk uuid="{78a3a6a1-ade6-41bf-8b29-ec7cf16de4bb}" location="/ram/overlay_ubuntu_16_04_64.vdi" format="VDI" autoReset="true"/> </HardDisk> </HardDisks>
You may retrieve the UUID of an VDI via
vboxmanage showhdinfo <vdi file> | grep ^UUID
on any Linux machine.
Greets