If you want to copy a layer mask in Photoshop, you need to use the Clone Stamp tool. The Clone Stamp tool is located at the bottom of the Tools palette. To use the Clone Stamp tool, click on the clone stamp icon in the top left corner of the Tools palette, and then drag it over the layer you want to copy. The clone stamp will create a new layer that is exactly like the original layer, except that it has a copy of the mask applied to it.
