If you can't export the RAW photos from the Photos app to the external hard drive, you can try to export it to a desktop folder, and then drag-drop to the folder to the external disk.
When you can't copy photos from Mac to external hard drive, the possible causes could be the permission issue on the external drive, incorrect files system, corrupted photos, etc. Therefore, one thing needs to be verified: do you upload photos from and export photos to the same external hard drive? If the answer is yes, there shouldn't be issues related to permission and file formats.
There is a workaround when you can't directly export from Mac Photos to an external hard drive, that is to create a folder on the Mac desktop, export the RAW photos from the Photos app to the new folder, and then you can transfer the folder from Mac to the external hard drive.