I discovered special thing. If I change anything on navi_sw cd , that MK3 cd informal. I change something - logo in navboot for MK4 (MK4 will receive and everything OK) and MK3 CD informal. Only write " In navigation computer SW - CD ........etc." and then black screen, will go out red diode flashing and nothing. No message "CRC BAD" or"CD bad" or someother.( I changed only navboot for mk4!!) ) Unique. I Made attempt. MK3 - changed modul in usw_load, computed CRC, everything ok, insert to the MK4 - everything OK, insert to the MK3 - write " In navigation....etc....." and then black screen, go out red diode and nothing. I did more attempts - if I wherever anything changed and computed CRC on OK, that MK4 will accept and MK3 not. Knows somebody wherein can be problem?