Externships can help you or hurt you. If you are a hard working, easy to get along with person, the externship will most likely help. If you have weird quirks, are lazy, or, in general, piss people off, it could be the end of your chances at that program. Most people can pull off a charade with their personality for an interview day; few can pull it off for a month. If you are not a likeable person, it will be revealed. Not trying to be mean, just saying how it is.
That being said, a good externship rotation will rarely overcome big deficiencies in your application. People may really like you as a person but still not want to take a risk on you as a resident if they are worried that you will not cut the mustard academically.