If the current partnership between GE & Safran is still the same as the old, a simplified description of responsibilities would be: GE is responsible for the core of the engine (compressor combustor & HP turbine) and Safran is responsible for the fan, LPT and gearbox. Then, modules are exchanged so engines are assembled and tested in both France and the US. I have heard the problem has to do with a manufacturing defect in one of the LPT disks involving one of two suppliers that Safran use for this disk. Don't know for sure this is a fact or not...