After smart resym messed up mesh on one side I decided to copy the good hand on the other side in external app. When re inporting athough the model has the same poly count zbrush refuses to devide to higher detail level and produces and badly deformed mesh.
I tryed to see what caused it and even if I chop off a arm and re weld it to its exact position in external app ,zbrush still has this problem of going up to a higher sub level.
When you unweld/weld vertices and or add/delete geometry in an external program you are most probably changing the vertex order, which gives you the nasty result when going to higher subdivition levels. 
Hi, what is the vertex order and is there a way to resolve it?
Each vertex has its own name (a number). When you alter the geometry theese numbers can, and mostly do change. You don’t want that when working with subdivition levels.
If you dont want to remodel the other hand to look alike the good one you could make a displacement map for the good hand and copy it in the texture to the ugly hands uv space. Then go back in zbrush and apply the displacement to the uggly hand. If you have your uggly hand morph brushed first it should look kind of the same as the good hand 
worth a try.