I’ve been getting the following error on occasion when rendering my Final Cut Pro timeline(s): “File Error: The specified file is open and in use by this or another application.”
I figured out quickly that you can just hit OK, and your render will continue normally…but it’s still irritating.

Well, a good friend of mine sent me a link to the fix (and I don’t know why I didn’t search for it earlier). It seems that the problem occurs when Spotlight tries to index your render files at the same time that FCP attempts to access them.

To prevent this from happening, you can tell Spotlight not to index the Render Files folder in your Scratch Disk:
1. From the Apple menu, choose System Preferences.
2. From the View menu, choose Spotlight.
3. Click the Privacy tab to reveal the areas that Spotlight will not index.
4. In the Finder, open your “Final Cut Pro Documents” folder.
5. Drag the Render Files folder into Spotlight’s list