After constantly having to re-learn the counter-intuitive, and failing process of installing maps to my Mac and Zumo every time there's a map update, I decided to write my step-by-step down.
I figured I'd share it here since map updates on the Mac are not only confusing, but the "simple" methods and applications offered by Garmin don't allow you to install to an SD, which, is the only place you can install them.
The attached summary is the only way I've been able to successfully update maps on my Mac and Zumo every time. It's a tedious process, but it's stable, and it works every time.
If some of you have had success in other ways, or find something I've overlooked, please let me know.