Q: Aperture 3: is it possible to have a library on an external HD?

That is no problem - you can have your library on an external drive. Simply move it there using the Finder. Then open it in Aperture in its new location by double-clicking it.

Only pick the fastest drive you have and make sure it is formatted MacOS X extended and directly connected, not a network volume.

See:

Aperture: Use locally mounted Mac OS X Extended volumes for your Aperture library:

http://support.apple.com/kb/TS3252

Regards

Léonie

Sure you can, just drag/copy it to the external drive and double-click on it to open it from there.

OTOH, I encourage you to relocate your master images to the external drive and keep the Library on the MacBook Air. Then you can look at your Library while disconnected and only need to reconnect it to edit images.

While disconnected, you can still import (and edit) new images in the Air's Library, then once you reconnect the external drive, you can relocate the master/original files to the external drive.

Of course, the usual disclaimer, "backup your library", applies here. If you have enough space on the external drive, or a different external drive, copy the original Library there before you experiment with relocating the original images.

In any of the above cases, you can send new image files to a drive of your choice in the Import window.

(While I was typing, leonieDF just said more-or-less the same thing, including an appropriate warning about the external disk's format.)