If you're importing images from an external disk and you select a folder on that disk, it may seem like you aren't able to subsequently navigate back up to the root level of that disk. However, you can, using different navigation path within the Import window.
The reason it seems this way is that Aperture remembers the last used path for an external disk when you select it by clicking its icon. In this picture, for example, Aperture remembers that the user imported from the folder "Stuff" the last time she imported from the disk "Aperture":
In this situation, clicking the icon for the disk Aperture in the Import panel simply toggles the Import dialog.
To get the desired effect, navigate to the external disk via the Local Files icon in the Import panel, like this:
Using this method, you can quickly navigate back to the root of the disk.