How do I change the UUID of a virtual disk?

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.

The following worked for me:

1. run VBoxManage internalcommands sethduuid "VDI/VMDK file" twice (the first time is just to conveniently generate an UUID, you could use any other UUID generation method instead)

2. open the .vbox file in a text editor

3. replace the UUID found in Machine uuid="{...}" with the UUID you got when you ran sethduuid the first time

4. replace the UUID found in HardDisk uuid="{...}" and in Image uuid="{}" (towards the end) with the UUID you got when you ran sethduuid the second time

The correct command is the following one.

VBoxManage internalcommands sethduuid "/home/user/VirtualBox VMs/drupal/drupal.vhd"

The path for the virtual disk contains a space, so it must be enclosed in double quotes to avoid it is parsed as two parameters.