How do you use Aperture on a separate hard drive - when you have too many photos for your computer hard drive?

Move your Aperture library to an external disk drive

http://computers.tutsplus.com/tutorials/how-to-move-your-itunes-iphoto-or-apertu re-library-to-an-external-drive--mac-45572

Allan

Many users come to the point of outgrowing the storage capacities of their system drives.

First and often overlooked: weed your garden.

More helpful:

You select one of two paths forward: put your library on a external drive with a satisfactory throughput (recommended: USB3 or Thunderbolt), or move some (or all) Images' Originals to an external drive.

Each of these is covered in the User Manual, in more detail than I can provide here.

When the library file becomes large and requires more disk space than is available in its current location, you can move it to a hard disk with greater storage capacity. To change the location of the library, you move the library file to a new location and then double-click it to open it in Aperture.

What Are Managed Images and Referenced Images?

In the first case, just move your Library using Finder, and then double-click it to open it and let Aperture know where it is.

In the second case, select the Images whose Originals you want to move from inside your Library to outside the Library, and run "File ▹ Relocate Originals ... ".

Search the forum for posts on both topics. These concerns are faced by almost all Aperturistas, and engender regular discussions.