I have built a new VM recently using VirtualBox and needed to use it as my base image, I stop the VM and copied the disk files to a new name and location. I then tried to create a new VM and attach the copied disk, only to get the following error

“Failed to open the hard disk”

“Cannot register the hard disk ‘testdisk2.vdi’ with UUID {UUID} because a disk  ‘Testdisk1.vdi’ with {UUID} already exists in the media registry””

The solution is to use the command line tool to clone the drive as follows:

open a command window (start > Run > cmd)

cd C:\Program Files\Oracle\VirtualBox             (move to the program files folder)

VBoxManage.exe clonevdi “testdisk1.vdi” “f:\VirtualBox\testdisk2.vdi”

This will clone the drive to the new file and location which you can then use. This will need to be done for each subsequent copy.