HI, Im trying to model from a poly-plane in Zbrush so I can create a normal map for a tileable rock texture.

I have turned off Transform>Modifiers>Xyz and changed the switches next to it so that only Z is active to clamp all editing to the Z axis, so it will be impossible to move points in the XY directions. This has enabled me to keep my poly-planes edge completely square. The problem is that I am working with a pre-made diffuse texture and when I switch the texture on which is mapped texture>UVP, it means I can no longer model with Zadd or Zsubtract. Any ideas, this seamed to be the ideal workflow to create base normal maps for tileable organic surfaces that I could enhance with smaller details in Crazy Bump. Any ideas?

Hmm. Could you try baking the texture into the actual polys of the mesh (txr>col) , then remove the texture and see if you can sculpt again like that?

That works a treat!