Miguel Martinez , Jan 27, 2011; 05:23 p.m.
I'd check the permissions of the folder where you are trying to copy stuff. As OS X is a certified unix, you need rwx permissions in ./original-folder/ to create a new folder inside there. Maybe someone or somewhat changed the permissions there. I suppose this is easy to check/fix in finder, and it is easy to do in a terminal.
If it's not that, check that you have enough space in the hard drive. Else... I'm lost, as I've never used a mac.