My Passport 1Tb external HD fails to complete the back-up

[RobertDW]

The back up process has failed every time I've run it. It goes no further than 97% complete. It shows that the back-up was unsuccessful. What is causing MY Passport hard drive from completing the process?

 

[popatim]

Which backup program are you using?
Which external drive do you have (model#) and how full is it?
Describe your system including which OS you are running.
What is the error message details?