The flying sorcerers. If earth ever contacts Pern again since they lost it in the files thinking the world is uninhabited and dangerous. I have vague memories that I should know more like this. Basically the 40's, 50's, and 60's had a lot of lost colony books. OH I know Andre Norton ummm , Star Born where the descendents of the scientists who escaped the purges on earth in The Stars are Ours are found and contacted by Earth.