Cherrie,

I want to be sure I understand exactly what you have, and where. You said that you have both the application and library on an external drive. Are you sure the application is on an external drive? The application has the icon of a lens straight-on, is named “Aperture”, and nothing else.

The application is probably actually in your Applications folder—which is where it should be.

The Library, on the other hand, is an icon that looks like a stack of three pictures. That is where all your photos are likely stored (there are other methods of storing photos, but unless the terms “referenced” and “managed” mean anything to you, I think it’s safe to assume your photos are in your Library).

The Library can live anywhere, and that’s probably what’s on your external drive.

Since it sounds like you have two things on your external drive though, I’m quite curious to what those are. If the application is there, you must have moved it at some point? Really, applications should live and stay in the Applications folder.

Regardless, there are several ways to back up your Library. The easiest way is to use Aperture’s own built-in Vault. I have an extensive article on setting up a Vault, and on understanding what is and isn’t backed up, here: Aperture 3 Vaults; What’s Backed Up, and What Isn’t?.

You can also back up using the built-in Time Machine, which backs up to a local external hard drive (however I believe it will only back up the internal hard drive, not an external one).

You can also use a system like Backblaze which will automatically back everything up to the cloud.

Personally I utilize both Backblaze and the Vault for redundant back-ups, both local and off-site.

Please do clarify what exactly is on the external drive for me, and you also may want to consider reading my “In-Depth Getting Started with Aperture 3” eBook. This will ensure you understand how the Library is managed, and where your photos are—as well as a host of other things.

-Joseph @ApertureExpert