You may consider to go for a mixed library scheme:
- Relocate the master image files of the older projects to your large external drive. (File -> Relocate master)
- The remaining library, much smaller now after you relocated most of the masters) move to your SSD - you do not need to create a new library for that.
After you move most of your library to an external disk and you once again fill up your internal drive, how do you move those "newer" images to your external drive?
Let's assume, you routinely import new pictures from new shoots as managed master image files into your Aperture Library on the SSD. You work with them a bit, tag the the images etc. Then you want to move them to the external drive.
You can at any time decide to turn any subset of your managed images into a referenced images by "relocate masters" . Just select the images you want to store elsewhere and again go to the File menu and select "Relocate Master". Just remember never to move any referenced master image files from the Finder, always use Aperture to do it, otherwise Aperture will loose track of their location.
If you want to move a master image file back into the Library, select "Relocate Master" instead. You can always tell by the badge overlays if an image is referenced or managed, see How Badge Overlays Appear in Aperture:
A referenced master will be marked by an arrow.
To quickly find all images that are managed, e.g., if you want to decide which images to move to an external next, create a smart album:
File -> New -> Smart album, click "Add Rule" and add a rule "File Status is Managed" .
To find all referenced masters use the rule "File Status is Referenced". You can also add these rules in any search panel, if you are using the search fields.