Er yeah, I did mean "ready boost". You could try running "disc clean", or downloading "CCleaner". That might point out any temp files being created during this process.
I like to do things the hard way, so I might allow DVD Decrypter to save the file to the HDD, then use the right click, "send to" command to copy it to the USB drive, and delete the original file.
Just out of idle curiosity, if you created another file with DVD Decrypter, would you lose another 2GB of disc space?