How to view photos on external hardrive

My wife has a zillion pictures on her iPad 2 that she would like to delete. All the pics/videos have been downloaded in the iPhoto app on our iMac. We also have an external hard drive on the iMac. She does not want to delete the pics on her iPad until she can see them on the back up hardrive. How does one view those files on the back up hard drive? The back up hard drive brand is Seagate.