Photo transfer from max desktop to a Seagate external hard drive

Are you using the Photos.app on your Desktop iMac?

If you want to continue using Photos it would be better not to export your photos but to copy the Photos Library.photoslibrary from your Pictures folder to the external drive. Photos needs a library to be arble to manage the photos. And the external drive needs to be compatible with Photos and prepared as described here: Move your Photos library to save space on your Mac - Apple Support

If you just export the photos you will have to use the Finder to browse and manage them.

And you should be sure to export the unmodified originals to preserve them in the highest possible qualityt and also to export the edited versions in a high quality. The File > Export commands offers two kinds of exporting - the unmodified originals and the edited versions. Use them both, if our photos have adjustments applied.