Good news - problem solved. The problem is when changing the firmware version to work with DCC-EX using the using the ST-linkUpgrade.exe by default this makes 3 changes "STM32 + mass storage + vcp", you wont see this unless you click on "change type". The reason this is a problem is in Arduino when selecting the correct board ie Nucleo-64 selecting correct board ie in my case "STM32F446RE" the "upload method defaulted to "mass storage". This should have been STM32CubeProgrammer (SWD) . everything is now working correctly. Many thanks for your support and hope my findings will help others