WD Backup - always starts a full backup

I have two My Passport Ultras. I was using Smartware but it was completely unreliable, and would leave files not backed up. A couple of weeks ago, I removed it and installed WD Backup. It seemed to get the job done as far as backing up the files, but took a long time to accomplish. I can live with that.

I do not leave my drives attached…it’s just not an option. So over the weekend, I reattached one of the drives to back up whatever might have changed over time, and when I checked an hour later, it was starting a completely new backup. It’s a 2 TB drive, and at this rate the third backup won’t fit.

Isn’t this software only supposed to backup the additional or changed items? I think it’s called incremental…

If what it’s doing is normal behavior, I need to make another backup plan or buy a different brand of drive. Help???