If you set the location of your Aperture Library to an external volume, you may later find that the location is unexpectedly reset to ~/Pictures/Aperture.
This happens if the volume you selected is unavailable to the system at the time you launch Aperture (for example, when the external drive is powered off).
If this happens, quit Aperture, make sure that the drive is connected and powered on, the relaunch Aperture. Check preferences in Aperture to make sure that your location is still selected.