If it is actually copying stuff, which I doubt after that long, I would suggest that you try Force Quitting the Finder.
Apple + Option + Esc. (oh, I just remembered, I got shouted at for telling someone that a while ago!) With any luck you will be able to relaunch the Finder.
Given that you did not win the PowerBall on Wednesday, your luck is not that good. So, if the Finder will not relaunch correctly, hold down the power button on the G4 until it shuts down.
Sit back, wipe your brow, cross your fingers and start it up again. It may take a while. All you original data should still be there as this was only a copy and not a move. You may need to tidy up the desktop of the other drive.
Let us know how you got on
I need to innterupt or cancel a massive file copy, gone wrong. On an old G4 PowerMac running OS X, I was trying to move about 1600 files by drag and drop. There was a lag that somehow got me to drop on the Desktop rather than the directory I wanted. The Desktop is on a different disc, and so the move became a copy, that has been running for DAYS! I have tried "Esc" and "Cmd"-period- but they don't seem to break the process. It is tying up my mouse/GUI, so I am looking for keyboard solutions. Is MrMacFixit out there?