See this: Macintosh System Update insert. Dated January 25, 1984.
Evidently the ghost disk was originally used to copy files in the one-disk system. e.g. drag your files to the unknown disk image and you are prompted to insert the disk you want to copy to, then the unknown disk becomes whatever disk you insert and the files copy, most likely the disk was auto-ejected when finished and the unknown disk returned to the desktop. Obviously this was a bad idea they quickly changed.[/url]