how do I store Aperture on an external hard drive?

SEB575 wrote:

Suggestions for storing my Aperture library on an external hard drive?

Drag and drop the Aperture library to an external drive, then double click it to open it in Aperture.

Just make sure, that the external drive has a compatible file system - MacOS Extended (Journaled) , and use a locally mounted drive, not a NAS:

See Apple's help document: Use locally mounted Mac OS X Extended volumes for your Aperture library