In "Time's Arrow," the discovery of Data's head in a cave is evidence that Data is to die at some point. During a conversation with Geordi in Ten Forward, Data tells him that such a revelation doesn't bother him in the slightest; outliving all of his friends would only reinforce the notion that he is artificial. Finally, he concludes that the finding of his head shows that he's mortal.
However, unless Data, for some strange reason, thought his head popping off would result from "android old age," then I'm not sure why this discovery would make him question his "immortality." After all, Data could have died previously by many different methods: Q could've destroyed him, the ship could've exploded, a phaser set to vaporize, etc. Data knew that he was mortal in this sense. So, I don't understand what his point to Geordi is.