Exporting photos from iPhoto to external hard drive

What is the file system format of the external volume? is it MacOS Extended (Journaled) or APFS or another format? Some things you could try:

• If the file system format is not MacOS Extended (Journaled) or APFS, do not export to the top level, but export to a sub folder. Some file systems do not support thousand of items at the top level of the volume.
• And export in smaller groups of files, not thousands of photos in one go. Then you can easier spot, which file is causing the problem.
• Export the files with the export setting "File > Export > Export unmodified Original": if you export the unmodified originals, Photos does not need to process the items and created the modified versions. You may have some items in your Photos Library, imported by iPhoto, that cannot be processed by Photos 5 on Catalina. The support for some legacy video formats and image formats is no longer available in Catalina. trying to export the edited versions of such items will give an error message, because Photos cannot process these items. These items should have been converted to a compatible format while still running a system version that supports them. Here is a list, written for iMovie: About incompatible media in iMovie for macOS - Apple Support But it it can give you an idea, which formats to avoid in Photos.