How To “Unerase”
You probably already know that you can use the History Brush (Y) as an “undo” on a brush, and that by default, the History Brush paints back to how your image looked when you first opened it. But did you know that the Eraser tool has a similar function? That’s right; the next time you’ve got the Eraser tool (E) active, look up in the Options Bar and you’ll see a checkbox for Erase to History. Normally, the Eraser tool erases to your Background color, but when you turn on this checkbox, it erases back to what the image looked like when you opened it.