Back up failure

The back up on my passport has completed and left my 500 gb drive with 2.8 gb of free space, the problem is that i dont i dont even have 500 gb of files. so what has happened and how do i fix it?

probably copies of changed files, by default it saves 5 different copies of things as they change. so like:

text01.txt changed 1 weeks ago 5kb\ will be one copy

text01.txt changed 5 minutes ago 10kb\ will be another copy