Change default action for copying files to external hard disk
You can move a file rather than copying to another volume (network, external drive, etc.) by simply holding the ⌘ Command key down while dragging the file.

If you are on the samve volume (EG you want to copy to another folder) hold down the ⌥ Option key will dragging and that will copy a file from any location to any other location on the same volume.