iannis@giannis-laptop:~$ VBoxManage internalcommands setvdiuuid /path/to/virtualdisk.vdi
VirtualBox Command Line Management Interface Version 2.2.2
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.
UUID changed to: 9e89fe14-d010-469e-a737-cd65218c4acb
Since the old UUID is replaced with a new one, you can now add and use the virtual disk.
Please note, that you wouldn’t have to follow this procedure if you had used the clonevdi function to copy the virtual disk image, in the first place. The clonevdi function makes sure that the new disk image will have its own unique UUID.