[dnanian]

I can only guess that your external drive failed during the copy, but the OS didn't tell us, so the data kept being copied to the internal drive's "mount point".

Please use Finder's "Go To Folder" command to open the hidden /Volumes folder. Look there to see if there's a folder with the name of the image or external drive. If so, delete that and empty your trash and the space will return.

If you can provide me with more information about your external drive (make/brand), its format, what else is connected, etc, that would help. Also -- whether or not you're running AntiVirus software or similar...