Aperture Library Spanner automatically spans your Aperture library across multiple volumes.
By default, Aperture's library is confined to a single volume. Consequently, when that volume fills up, you can't add any more images to your library. However, using a little Unix trickery, it's possible to span the Aperture library across multiple volumes to extend your storage. The Aperture Library Spanner is a simple app that will automatically span your library across multiple volumes.
You can span your library to an additional internal or removable drive. Using the Spanner application is simple and full documentation is included. Apple doesn't formally endorse spanning your library this way, and spanning to an external drive can create some hassles if you launch Aperture without having the drive mounted, but many users have been employing this technique without harm.
The Aperture Library Spanner is available for free, but please don't distribute it.