How can I restart Finder when it keeps "stopping" an stopped copy process?
Next time this happens, look at the Finder process using ps: First run this in a terminal:

launchctl list | grep Finder


The output should be just one line, listing com.apple.Finder with a number at the beginning of the line. That number is the process ID (pid) of Finder. Now run

ps up xxx


replacing xxx by the pid found in the first step. Look under the STAT heading. If you see the letter U then the Finder is stuck in an uninterruptible system call, and cannot be killed. You will most likely need to restart your system to recover.

Processes stuck in an uninterruptible system call is often a symptom of a hardware problem. It would bear further investigations to find the cause.