How do I migrate my Aperture library from my primary HDD to an external drive?
Yes you can, here are the steps:  


Connect your external disk
Open Finder and go to &lt;your home folder&gt;/Pictures
move the Aperture Library.aplibrary file to a location on your new disk
Open Aperture and go to preferences ( from menu or by pressing ⌘+, )
On the General tab, change the location of the library to where you copied the Aperture Library.aplibrary file


That's it, the library is now on your external drive.