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I've been using Photoshop for years but this issue has me totally baffled. It must be some setting I changed, but I have NO IDEA why this happens or how to change it.
The issue is that on layers masks (doesn't matter what kind), the erase tool will ALWAYS leave some percentage of opacity, no matter what I try. You can see the vague grey-ish area that I attempted to erase, but it will never go to full transparancy. See image below.
Don't use Erase on layer masks, just use the brush.
Pick a colour - well, two colours, white to paint in, black to paint out, grey if you need finer control [though I personally use the opacity & flow controls instead of greys].
Use X to flip between the two quickly. 
Erase erases to the current background colour, which not only is a variable, just like the brush, but will also erase bits you didn't mean to, as it indiscriminately erases both black & white [and any grey in between] back to itself.
