Aperture allows you to keep multiple library files, by simply changing the Library setting in Aperture’s Preferences dialog box. Many people use this as a way to get around the fact that Aperture limits the size of an individual Library to a single volume. For example, they’ll use one library for work files, and another for their personal projects.
While there are no right or wrong answers to how you should architect your Aperture library structure, I’m increasingly of the mind that it’s much better to have a single library. First, it simplifies backups. Second, it lets you take advantage of one of Aperture’s core features: a searchable library of your entire photographic archive. Third, multiple libraries introduce an extra level of organization and management hassle to the already complex chore of managing your image library.
So what do you do about that single volume library limitation? Fortunately, the ability to import images as references pretty much solves this problem, because your master files don’t have to consume any storage on the volume where your library is kept. What’s more, the previews that Aperture builds are small enough that you can have hundreds of thousands of them on a reasonable-sized drive. Because Aperture allows you to re-build previews, you can easily change your preview compression settings and rebuild them at any time, to free up more disk space.
Of course, if you keep everythingin a single library, then your Projects pane will be harder to navigate, as it will be crammed full of more projects. To minimize the clutter, consider grouping projects into Folders, or take advantage of Aperture’s Favorites menu. Tag all your personal projects as Favorites, and then change the All Projects menu to Favorites.
This allows you to easily switch to a select group of projects. Or, change the menu to Recent Projects to view the last 20 projects that you worked on.
Keeping one library lets you take full advantage of Aperture’s keywording and search features - features that end up somewhat less useful if you’ve spread your images across multiple libraries.