Multiple changed CD-R's - for no reason

I have a very large family history file on my computer which I periodically save to a CD-R. One disc has four versions saved, and another has one.

The other day I was trying to separate some of the file from the main bulk of names, and busily deleted many names. After I stopped for the night, I went from the copy (in which I had been working) and into the main file. To my horror it mirrored the copy. It no longer had all of my names.

I pulled out the two back-up files, containing a total of 5 saved versions and EVERY ONE MIRRORED THE COPY IN WHICH I HAD BEEN DELETING NAMES.

The back up files had been nowhere near my computer. Does anyone have any idea what happened? A relative is returning the disc I sent the other day, which should be good, and I'm almost afraid to touch it. How can I avoid this kind of contamination?

Thak you, Barbara McCusker
wildwoods@crusoe.net