you can no longer start on the corner of a box because as soon as you hover over the corner of a smart rectangle with the box tool (m) it converts your cursor into a transform tool and drags the existing box out of shape.
I hate the smart rectangles too, but in your case, all you have to do is turn on Smart Guides (⌘-U) and deselect the old rectangle (⌘-Shift-A) before starting the new one.