Once it registers and appears on the VM list (you should register the pvm on the external disk, not a copy on the desktop) you don't have to register it again, it stays on the VM list with a question mark when the external drive is not connected and when you connect it it recognizes it (unless the path has changed, or you changed the name of the Volume/Partition on that external device.
To access the VM list: launch parallels desktop.app, from the top menu: Window>Virtual Machines List
Last edited: Jul 30, 2012