If you've copied a disk (vmdk file) from one machine to another and need to change a disk's UUID in the copy, you don't need to change the Machine UUID as has been suggested by another answer.
All you need to do is to assign a new UUID to the disk image:
VBoxManage internalcommands sethduuid your-box-disk2.vmdk
UUID changed to: 5d34479f-5597-4b78-a1fa-94e200d16bbb
and then replace the old UUID with the newly generated one in two places in your *.vbox file
<MediaRegistry>
<HardDisks>
<HardDisk uuid="{5d34479f-5597-4b78-a1fa-94e200d16bbb}" location="box-disk2.vmdk" format="VMDK" type="Normal"/>
</HardDisks>
and in
<AttachedDevice type="HardDisk" hotpluggable="false" port="0" device="0">
<Image uuid="{5d34479f-5597-4b78-a1fa-94e200d16bbb}"/>
</AttachedDevice>
It worked for me for VirtualBox ver. 5.1.8 running on Mac OS X El Capitan.