Two days ago, I found what looks like powdered milk in a pile by my garage door. It is fluffy and dry and really oes look like powdered milk. I have seen carpenter ants, only one or two at a time, in the area. They are the big black variety. I can't see the hole or evidence of them coming and going. Do I have an ant problem, or is it more likely termites?

You might be seeing the frass of the powder post beetle. Check this link for a comprehensive description Powderpost Beetles | University of Kentucky Entomology (http://www.ca.uky.edu/entomology/entfacts/ef616.asp)