iPhoto Libary cannot repair - size: 500GB

What is the file system format of your external drive?

Prepare your external volume as described here:  Move your Photos library to save space on your Mac - Apple Support

The important points are: The filesystem needs to be MacOS Extended (Journaled) or APFS, the volume must not be used for Time Machine backups, the volume needs to be locally mounted (not a NAS or network share or a synced cloud folder).

You need to be able to set the ignore ownership flag, or the volume will not be suitable.

As for repairing the iPhoto Library:

Option 1

Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.

If that fails:

Option 2

Download iPhoto Library Manager and use its rebuild function. (In early versions of Library Manager it's the File -> Rebuild command. In later versions it's under the Library menu.) 

This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.

Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.