Independent of my questions above - the problem may be the file system of your external drive.
If the file system of the drive is not MacOS Extended (Journaled), newer versions of Aperture will not see libraries on the drive.
Try to copy one of your old Aperture projects to your Pictures folder. Then double click it to open in Aperture. Can Aperture now open it?
if not, look at this support document: Aperture 3.6 Release notes - Apple Support
Scroll down all the way to the last paragraph at the bottom - "Lear More".
Apple recommends to repair or rebuild a library in Aperture 1 or 2 before upgrading it. And before rebuilding it is essential to make a backup copy of the library.
since you can no longer run Aperture 1 or 2, try repairing in Aperture 3 instead. Keep a copy of your library on an external drive before you try this.
Select the copy of your old Aperture Library in your Pictures folder and hold down the Option and Command keys while double clicking your library. Keep holding down the keys firmly until you are seeing the First Aid panel. Select "Repair data base".
If you are upgrading your Aperture library from an earlier version of Aperture, here are some guidelines to assist you in a successful move to Aperture 3:
- Before upgrading, it is recommended (though not required) that you perform a rebuild of your Aperture library. To rebuild an Aperture library, hold down the Option and Command keys while opening your library in Aperture 1.5 or 2, then click the Rebuild Now button.
- Aperture temporarily uses extra space on your hard drive during the upgrade process. This is for the purpose of backing up critical library info and insuring the integrity of your data during upgrade. If you don't have adequate space on your hard drive to accommodate the upgrade, Aperture will display a warning dialog. You may need to move your library to a different hard drive with more space in order to upgrade it, and then move it back to the original drive when the upgrade is complete. Any space used by Aperture during the upgrade is released and made available to you again once the upgrade has finished.
- Keep in mind that fully upgrading a very large library is a one-time process that can take a long time; the process may take many hours or even overnight, depending on the size of the library. Once the initial database upgrade is complete, you can begin working with your Aperture library. However, post-upgrade processing will continue in the background for some time, as face detection and preview generation are performed. (A progress indicator and the word "Processing" will appear in the status line of the Viewer or Browser to indicate these operations are underway.) While you can work with Aperture while these operations are performed, you may experience temporary slow-downs. Optimal performance will return when all post-upgrade processing is complete.
