Dennis, there must be something else wrong with your Aperture Libraries, if you are getting this message, with so much free space available. Have your Aperture libraries been last used with an older Aperture version and need upgrading? How do you start Aperture? By clicking the Dock icon? Then try the following:
- If your Aperture Libraries need upgrading to a newer Aperture version, repair them first:
Select one of your Aperture Libraries on the external drive, hold down the key combination ⌥⌘ firmly, while you double click the library to launch into the First Aid Tools. Select "Repair Database". See, if Aperture now opens correctly.
- If your Aperture Libraries does not need upgrading to a newer Aperture version, simply double click it to open in Aperture.
Does any of this make Aperture launch properly on one of your libraries?
If not, try, if you at least can launch Aperture to create a new, empty Aperture library, by holding down the ⌥-key, while you launch Aperture and select "Other/New".
Regards
Léonie