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I was having this problem just now but i figured out why.

I was trying to drag a file into another file on my toshiba external hard drive. When i tried this it said there was not enough space to COPY it. Which is why i ended up here. I had to select the hard drive and go into "get info" and add myself to the permissions to ''read and write". BAM!!! problem solved. I drug the file right in with out coping it on the next try.
 
I was trying to drag a file into another file on my toshiba external hard drive. When i tried this it said there was not enough space to COPY it. Which is why i ended up here. I had to select the hard drive and go into "get info" and add myself to the permissions to ''read and write". BAM!!! problem solved. I drug the file right in with out coping it on the next try.

Are you saying that you were trying to move a file from one location (folder) to another on the same external hard drive? You say that you changed the permissions after you "had to select the hard drive". Presumably that is the external hard drive that you're referring to. So it appears that the problem was not space, but permissions, in addition to a not very helpful error message.
 
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