Step 9: Make Another Selection to Change Your Layer Mask
Finally, since the layer mask is still on the barrel layer, you can always make another selection and try again. This time, select the Rectangular Marquee tool (press Shift-M until you have it), make a square selection in the middle of the canvas, and fill it with black.
Photoshop doesn't care what shape you create. It just cares about black and white. That's just the beginning, though. Layer masks get way cooler. In fact, there's a way to automatically create a layer mask.
Tune into imaginginfo to find out how. Here's a link to the second in the series.
(Excerpted from Layers: The Complete Guide to Photoshop's Most Powerful Feature by Matt Kloskowski. Copyright © 2008. Used with permission of Pearson Education, Inc. and Peachpit Press.)