EASY FIX: Could Not Save File Already In Use Mac OS X
How to fix a very frustrating bug in OS X when you try and save a file. It all comes from the Finder's view option. If you have an item in "Gallery View" or preview when you use the space bar it will cause this problem.
I've noticed this happens when I am using image editing software but it can happen in any program. Good luck and let me know if it helps you