Increase the Size of a VirtualBox Hard Drive

To change the size of a disk, you need to clone it to vdi, increase the size, and then

VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
VBoxManage modifyhd "cloned.vdi" --resize 51200
VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk

To change the UUID of the disk, simply run this command:
VBoxManage internalcommands sethduuid disk2.vdi