External Hard Drive says its full but its not..?

I'm trying to move things from my C Drive to my external hard drive..I only have 3 GB left on my C drive.My external hard drive says it has 31 GB space left but when I try to move something from the C Drive to the external hard drive that's less (I tried with 5 GB item) it says there's no... show more I'm trying to move things from my C Drive to my external hard drive..I only have 3 GB left on my C drive.My external hard drive says it has 31 GB space left but when I try to move something from the C Drive to the external hard drive that's less (I tried with 5 GB item) it says there's no room...what's the problem and how do I fix it????
Update: Bjorn & Gary-I don't understand..I have 149 GB Hard Drive with 32 GB Free..chow could I not be able to move 5 GB files on there when I have before?

Say Word-That might be it..how do I remove the backup space?
Update 2: *how could I not be able to
Update 3: comtech-I ran CCleaner before I tried.
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