Failed to open the hard disk <FilePath>.
Cannot register the hard disk <FilePath><GUID> becuase a hard disk <DifferentFilePath> with UUID <GUID> already exists.
I looked all over the place to find what was causing it and finally found a fix. Of course it requires the command prompt, so open that junk up and let’s get started!
**Note that I am on VirtualBox version 4.1.6 and from what I’ve read this command used to be something different a few versions ago. I think instead of setdhuuid it was setvdiuuid, but don’t hold me to that.**
Open the Command Prompt
Change the directory to where VirtualBox is installed (Default: C:\Program Files\Oracle\VirtualBox)
Type the following VBOXMANAGE.EXE internalcommands sethduuid <PathOfNewVHD>
Note that internalcommands and sethduuid need to be lowercase. I spent more than a couple minutes trying to figure out why things weren’t working. 
Below is the output of me changing the ID of the VHD located at C:\Users\bschacht\VirtualBox VMs\Windows XP (***) Copy.vhd
C:\Program Files\Oracle\VirtualBox>VBoxManage.exe internalcommands sethduuid F:
\os6.6-2\OS6.6.vdi
UUID changed to: 6b986826-2006-47d5-9ef1-ac85cea99157