Josje,
Welcome to the Apple Discussions.
Do a Finder search for a file called "Library.iPhoto". This is the database file created for each working library. I'd bet that you have one on your external drive, belonging to the lost library, and one on your internal drive, because you said you can see your most recent imports with your external drive unmounted. (iPhoto couldn't find the library on your external drive, so it created a new library on your internal drive. That's where your most recent imports are being stored.)
When your search gives you the Library.iPhoto files, click once to see the folder path of each one. That
file should be contained in a
folder called "iPhoto Library" which is the
folder that you want to locate in order to move your library back to a known location. Look at the folder path to see where the library was placed - maybe it will help you figure out what happened. You can then use the Finder to navigate to the location of the iPhoto Library folder, and move the entire folder to your desired location. (Move the folder as an entity; do not open it or move parts of it.)
To keep a library on your internal drive, locate the one that is there now and move it to your Pictures folder. If you don't want it on your internal drive, rename the library folder (ex: iPhoto Library Recent) and then copy it to the external hard drive. Don't place it inside the other Library folder, place it in the same location as the other so that the two will appear together. Later, when you've checked that all is well, you can delete the Library folder on your internal drive.
To point iPhoto to a library, hold down the Option key as you launch iPhoto. Select "Choose (or Find) Library" then navigate to the Library folder that you want to open. iPhoto opens the previously selected library by default until you Option-launch and change it.
You have 2 choices for how to combine the libraries. The only way to move photos between libraries and preserve albums, keywords, and other metadata is to use
iPhoto Library Manager. This will allow you to merge the new, small library into the other one. If you haven't spent much time editing, organizing, and keywording the photos in the new library, then you can simply import the Originals folder from the small library into the large library and begin organizing them from there.
Regards.
