How do I relocate a existing VM with a OS already installed
As you're moving the VM's location but not the hardware that the VM is running on (i.e. you're not changing the host computer), moving the files is fine. This is only fine because of this clause. Moving the VM to a different host computer should be done with import/export.

You need to make sure that you've moved all the files, otherwise errors will occur, as well as making sure that you move rather than copy the files, otherwise you may get duplicate UUID errors. Deleting the VM from VirtualBox without the external drive attached reduces the likelihood of duplicate UUID errors, as follows:


Quit VirtualBox.
Move the VM files to the external drive.
Disconnect the external drive.
Open VirtualBox and remove the VM.
Attach the external drive and add the VM to VirtualBox.