For a separate building, such as a detached garage, you are required to have a disconnecting means on the building. In this area, I typically have a 'safety switch' mounted on the outside, feeding the panel directly behind it (on the inside). A 'main breaker' in the panel is another way to do it- but is usually more expensive- and our AHJ likes the disconnect on the outside anyway.
There is one exception- no such disconnect is necessary if you're running only one circuit- and "circuit" is vague enough to include a single multi-wire branch circuit (or what we normally think of as two circuits). Assuming that you are using both full-size breaker spaces for a pair of full size breakers, you might be able to argue that you have but a single multi-wire circuit, with the breakers simpling being an additional, not required, disconnecting means.
