The following is what to do if the seagate is NOT a time machine backup. Plug the drive in. Once it shows up on the desktop, hold down the option key and click the photos flower icon. A dialog will appear asking you which library you want to open. If the library on the seagate is not listed, click browse and navigate to your iPhoto library.
Alternately, if you have the room, you can drag the iPhoto Library to your desktop from the seagate and then hold option and click the Photos icon.
If this is a time machine backup, you will need to use the time machine interface to restore the library.
Restore items backed up with Time Machine on Mac - Apple Support