low resolution photos and videos won't play after moving photo library

The syncing with Dropbox may have damaged the Photos Library. It is not safe to put a photo library on a volume that is syncing with a cloud service. It can break the links between the versions and the original files and the database files. Apple warns against it in this document: Updating from iPhoto to Photos for macOS – Apple Support

 If you store your Photos library in a location that's part of a cloud service (for example, Dropbox, Box, or Google Drive), your library might be damaged by the syncing process. As a result, you could lose photos.

To see your photos on all your devices, you can use iCloud Photo Library. To back up your library, you can use Time Machine.

Do you have a backup copy of your library, from before you put it into the DropBox?

I have never used a Seagate S Drive, sorry. I hope Keith knows more about this.

Usually the way you moved the library to the external drive should work to move them back.

If the file system format of the backup drive is not MacOS Extended (Journaled) you might get copy errors, however.

What happens when you double click on an image?

If it was just a copy of the library, you should be able to drag it back.