Aperture: Library over multiple Hard Drives
This is a very easy operation that basically is one menu item away.


File -> Relocate Originals…


So, just connect whatever drive you want to store some photos on and select those photos (by project, album, smart folder, whatever) and then relocate them.

A standard file dialog will appear and you can move them to whatever folder you want on the other drive(s).

Aperture will then update the database and move the large / original version of each photo to the drive selected. When you are using aperture and the drive is not connected, you still have a preview version to work on / edit / crop / correct / etc… If you perform some operation where the original is needed, Aperture will ask nicely that you connect the drive, but for routine tasks - you may not even need the original version for daily use.