Seagate external hard drive.

1. Where are the photos stored on the computer?
2. Which app do you use to manage your photos?
3. Which app do you use to back up your computer?
4. Why don't you think the photos are not being backed up?

More details: are you getting an error on backing up photos? how big is your backup drive? How are you backing up?

You only answered 1 question, the others need to be answered too please. If you do not answer the questions there is not much I can do to help you.