Thanks for the suggestions, but as I said I can load one HEX file, place a 12mHz xtal in the development board and click on connect in the serial tool and it works fine. Disconnect the port, load up the other code, replace the 12mhz xtal with a 20 mhz with identical port setting in the serial tool (and those that match the settings in the code) click on connect and get the "unable to open com port" type message.

Settings in device manager mimic those set in the code. No issue reported, driver provided by MS