The 645m uses MTP, and by default will show up as a device and not a drive under windows. There are some 3rd party apps that will map an MTP device as a drive but I've not used any.
Thanks, Jim. Turns out I don't need this after all. What I was trying to do involves my use of SportTracks and the Garmin FIT plugin that's no longer supported. I since found a way to sync everything from Connect to SportTracks Mobi to my desktop SportTracks automatically, which is even easier than the way I've been doing it.