1tb Passport won't backup my partially used 1TB computer-WHY?

I have a 1TB My Passport Ultra external hard drive that I completely format so that it is completely clean. I have no trouble storing all of my videos, photos and music on it and that is what I use it for. Well, if I format it again so that it is completely clean and run a total backup from my computer for everything, I get a notice that I don’t have enough space in my external hard drive to do a complete backup of all of my data and my OS. What I can’t understand is that when I check my computer, I am only using 555GB on my 1TB hard drive. My operating system takes up only 20GB of space so that my 1TB external hard drive should be sufficient to backup both my data and my OS. Can anyone explain what is happening that my computer running Win 10 is telling me that my external hard drive is too small to do a complete backup? Thank you.

Which application is the one you use to handle your backups? Does this happen with other external hard drives?

I just used the Microsoft backup. Win 10. Latest development–I formatted my 1GB external hard drive again and started a completely new backup of my entire computer. This time it worked and I was even able to make a Win 10, 64 bit repair disc as well. Unfortunately, when the backup was completed, I was not able to open any of the line items that were shown in the box that popped-up. Then, I uploaded all of my photos, videos and music to the same external hard drive and now when I click on that external hard drive, all of my photos, music and videos come up but none of the line items that were present in the box after my initial backup. Strange!

Sent from my new 6th Generation iPod (Gold Edition) :calling: