Making copy of the .vdi file into another location will make a real copy of the virtual disk, including the UUID of the disk. If this disk is added to the Virtual Media Manager, you will get an error like this:
To chage the UUID of disk use following command
$ 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-cd6521889acb
Since the old UUID is change , you can now add this virtual disk to Disk Manager
You need not use this commmad if you use clonevdi function to duplicate the virtual disk image, in the first place. The clonevdi makes the new disk image with unique UUID.
The syntax of the clonedvi goes like this:
$ VBoxManage clonevdi_Master.vdi Clone.vdi