Ok. Follow these instructions and it should help. By the way Windows backup isnt for backing up to external drives. Each comuter comes with a mini storage file on your main hard drive then when using W.B the files go to that folder once the folder is filled you cant use it anymore. W.B is meant for only backing up system images not files.
Instructions:
1) Format the drive. Plug in the drive>go to the drive in My PC>right click the drive>click format>then format the drive.
2) Look through this article and find which program you want to use then install it. LINK:
http://www.techradar.com/us/news/software/applications/best-free-backup-software-11-programs-we-recommend-1137924
3) Setup the software and you should be good!