It is a copy of Aperture library I presume, as it is about 1 gig in size.
Check it in the Finder. Does the copy on the external drive show an icon like an Aperture library and has the filename extension ".aplibrary"?
Like this?

Or is the filename extension ".apvault" and are you seeing a vault icon?

If it is the Aperture Library icon, you should be able to open the copy in Aperture by double clicking it to launch Aperture on this copied library.
If it is the vault icon, you can change the ".apvault" filename extension to ".aplibrary" in the "File > Get Info" panel for the vault and then open it like an Aperture library.