I was detailing a creature in zbrush and i wanted to mirror/duplicate the wings. The wings already had details so I had to mirror it at the highest subdiv level. And i wanted it to be appended as a separate object. However, when i go to the mirror object or duplicate, i can subdivide it further but the reconstruct subdiv operation won’t complete. It’s not disabled, but it just returns an error, “cannot reconstruct subdiv, operation failed” or something. The original object didn’t have tris, and since i was just using the mirror command using subtool master, i would assume that the object would be identical to the original one. ANy help would be appreciated. Thanks! 
check to see if the object has UVs Enabled. If the mirror doesn’t have UV’s, it won’t reconstruct.
thank you. I had the same problem and i would have never find out that this is the solution.