Stuck on "Preparing to copy"

I tried to copy a bunch of pics from an external HD to my Mac, and the Finder window is just stuck saying "Preparing copy to Pictures." I can't end it by hitting the little "x" and I can't even shut downs the computer because it tells me something like Finder windows it open or something like that.

I don't want to mess up the pics on the external hard drive either, so what is the correct way to stop this and maybe try again? Should I just switch off the exernal hard drive? Just inplug it from the USB? Like I said, I would hate to lose any pictures or anything.

Un-powering the ext. drive (Especially during a file transfer) could do some serious damage and loss of data.
Go to the Apple menu-force quit- and force quit (relaunch) the finder. It's the safest option you have.

Remember, when doing a copy, Finder traverses the entire tree to count items and size it up. The more you have to move, the longer it takes.

Remote possibility: the external drive may have a corrupt filesystem on it. Check/verify it first.

Thanks for the help. I am glad I know about the Force Quit now. Sometime overnight the Preparing to copy window went away, nothing was copied though. But I just tried it again and it works fine. Thanks again.