How to import a folder with 500GB files export from iPhoto to Photo

Are you trying to import the whole iPhoto library? This is not the correct way, and will, as a minimum, cause the problems you are seeing.

The correct way is to convert the iPhoto library to a Photos library by opening it in Photos.

If you don't already have a backup of the iPhoto library, make sure you take one first (the migration can (rarely) go wrong.

When you have a secure backup of the library, open the photos app, while holding down the option key. If the iPhoto library is shown in the resulting list, select it and click open.

If it is not, click "other library" and locate it from the browser. The migration will start automatically.

Bear in mind - depending on the size of your library it might take a long time to update, and sometimes the progress indicator doesn't move for a long time. You'll need to be patient, AND leave your Mac on (connected to power) until it completes.

You will then have two libraries on your system, your current library, and the converted iPhoto library. If you choose, you can merge them using one of the options described at the link below. Personally I use Power Photos.

https://discussions.apple.com/docs/DOC-250000662

I second Tony's recommendation to convert the original iPhoto Library instead of exporting from iPhoto and then importing the iPhoto photos as individual files. By exporting and reimporting are you losing the connection between the originals and the edited versions, all your albums and events.

Preparing 500 GB of photos for import will take a very long time. Photos has to compare the photos to the photos already in the library and to create thumbnails to show them in the browser. And you may be running out of memory, if Photos has to cache the thumbnails for thousands of new photos for import.

If you only have the exported files, import them not all at once but in small groups. This way you can see more easily, if Photos encounters image files or videos in a format, that is no longer supported by your current system version. iPhoto supported different media formats from the current version of Photos. You need to detect these files and weed them out and convert them to a compatible format.