I have a passport hard drive to use in storing files and photos, it works on my windows PC, but the Mac does not allow me to add files or change anything on the passport. Can you help me with this problem?

While a Mac can normally read a disk with most any format, it may not be able to write to a PC formatted disk. You may have to first save the files to another medium and reformat the disk.

Maybe, helmyn.... what Mac OS do you have/are you running on your Mac?? when you have your passport hard drive plugged into your Mac, click on it once, and press Apple+I keys together (get info) and you should see a long window, about halfway down, you'll see "Sharing and Permissions" ........2 columns, if you click on one side, you'll see the user and the other side, their sharing and permissions...... if you're running Catalina (10.15), the default is "read only" .....to change that, go to bottom of window, click on lock to unlock it, if prompted, type in your admin/login password and the lock should unlock, and you can change those permissions, etc, when done, click on lock to save changes, that may fix your problem

John B