NonStop Backup - External USB Drive Greyed Out

I have two external USB-connected drives and when I go into the NonStop backup setup screen where you can pick which drives you want backed up every 5 minutes . . . BOTH of the External Drives are selectable but the "Protect" button is greyed out. I do not understand.

I need to backup my smaller, 120 GB USB Drive constantly since it contains all my Work files. Any help ??