Won't stop backing up on time machine until error happens

I have a 3tb extenal hard drive. I formatted like the web site said.  I have 2.25 TB of infromation to back up on a 3tb drive.  I am using time machine. Just when it is about to finish , it starts to recalclute the finishing time.  Then it exceeds the amout of storage on the hard drive and shuts down.  I have no idea why it is doing this.



Have you tried backing up to another drive to verify if it is not a drive issue but a software issue?