VirtualBox Cannot register the hard disk already exists

1. Select File from Oracle VM VirtualBox Manager
2. Virtual Media Manager
3. Remove the file (highlighted yellow) from Hard disks tab.

I found a solution

File -> Virtual Media Manager -> Removed existing images (note, I removed them only from the registry).

I followed these steps.

http://www.webdesignblog.asia/software/virtualbox-moving-vdi-file-re-linking-guest/#sthash.1QOHeiw5.dpbs

After that I could update the path in the VM settings.

It's possible to change the UUID on the VDI file using the command:

 VBoxManage internalcommands sethduuid <file.vdi>

It seems this command assigns a new "random" UUID to the file. No much documentation on it, though.