I too have been researching Dow in Nova Scotia and I have never come across any McDow or MacDow. As far as I have been able to tell, Dow is a stand-alone Scottish name, from Dhu. When my ancestors came to Canada, they pronounced their name Doo, as I have a gif image of a deed to Robert Doo, birth records online for Dew, and some census takers spelled it Doe, but it is all the same family line Dow(e). Some members of my family started spelling it with the "e" back in the 1880s/1890s, for reasons known only to them, although my great uncle made vague notes about a family fight that led to some members wanting to not be associated with "the Dows" so they added the "e". There are many variants of the name though, and some people changed it on arrving in Canada. I also found that my Burnett line did NOT come from Scotland as was always presumed, but from Ireland, so name origin is not often as clear as Mc, Mac, or O.