Can't backup to a 2nd external hard drive

I just purchased a 2nd Passport so as to have two discrete backups of my files from my computer. My first passport works great. However, even though I can see my second Passport on the computer, WD Backup is not acknowledging that I have a new Passport. It keeps asking for my old Passport and hence I cannot backup files to the new Passport. I have put encryption/passwords on both.

Can anyone help me with this?

Thank you. -Kach