Copying frozen

I am attempting to move a large amount of files from one external HD to another. At first I moved the files in small batches, but the last very large batch has frozen the copying process. I get the window saying it's copying X number files and it will take 2 hours. That was 4+ hours ago. Clicking on "stop copying" does nothing. The computer won't let me restart or shut down either because the copying application is running. How can I stop the copying or at least reboot? HELP!