HOW TO MERGE MULTIPLE IPHOTO LIBRARIES INTO ONE ON EXTERNAL HARD DRIVE

Do you want to merge iPhoto Libraries or Photos Libraries? This is not quite clear, because you are asking in the forum for Photos, but mention iPhoto and not Photos.

And it will depend on the System version you are using.

If your Mac is running macOS 12 Monterey or macOS 13 Ventura, you can import the photos from your iPhoto Libraries or Photos Libraries into a copy of your largest library on your external drive.

See: Import photos from another library in Photos on Mac – Apple Support (UK)

This will not be a complete merging of the libraries, however. It has some limitations, as the keywords, albums, and folders will not be imported. The merging will avoid duplicates however, and keep the edited versions paired with the originals, when you are importing from a Photos Library. And when importing from an iPhoto Library only the originals will be imported. Only importing from a Photos Library will save the edited versions. So you may want to convert any remaining iPhoto Libraries to Photos Libraries.

You will get a much better merge result, if you merge all libraries in iCloud. Use iCloud Photos to store photos in iCloud – Apple Support (UK)

• Convert your iPhoto Libraries to Photos Libraries by opening them with Photos (this is no longer possible on macOS 13 Ventura however).
• Make your largest library an iCloud Photos Library, wait for the library to upload to iCloud, then make each of your other libraries your iCloud Photos Library in turn. iCloud Photos will merge the libraries in iCloud into one, keeping all albums, folders, metadata, sync the faces and the places, the keywords.

More ideas on merging libraries are here: Notes on Merging Photos Libraries, 2019 Version