Thanks Spycatcher 2K and Sterretje for your comments. I have changed the Board manager to ver 1.6.21 and used the 'old bootloader' option as you suggested to upload my sketch while selecting the Nano board. The sketch does not upload and reports the error I originally posted. I am not overly concerned as I can upload the sketch using the Uno option from the Tools drop down instead of Nano but I would like to find out why this is happening. If you have any other ideas I would like to hear them. 