With test light I can detect voltage on my desktop pc case and I believe it's the reason that my HDD fails some often or my pc hangs. When I connect my case to ground (metal door frame) it works with no problem.
I have disassembled my pc and found that the leak is from power supply unit (as you have guessed). I have replace the power supply with a new one but I still have the same problem.
Any idea how to repair it or what the problem is?!