The Mask Bias works great and really fast only if you want to duplicate objects from one layer to another hierarchically. However, it does not work if you want the objects on the target layer to be propagated to the top level, in another word, flat. If it's the latter you want, then you will have to use Generate Layer.

I would recommend that use “Mask Bias” operation. This is an hierarchical operation and is much faster than Generate layers operation. Please perform the following steps to copy geometry from layer “Diffusion B” to layer “Diffusion A”:
1. Open your top level cell.
2. Go to Tools>Add-Ins>Mask Bias.
3. Set up the Mask Bias dialog as shown below and then click on Drawn->Mask button.

Need to copy all objects layer “B” to new layer “A” on the top level.