Mac Passport Won't Overwrite Existing Back Up

I have a Passport for Macbook says it is out of space as it won’t automatically overwrite the existing file. I have a 500 GB hard drive (2/3 full) and a 500 GB back up drive. Why won’t the back up automatically overwrite the old file to back up? Is there a setting somehow? Help.


What backup software are you using?

Using time machine.