No, you shouldn’t put osx on your scratch disk, but you can still repair permissions your boot disk that has OSX. Quit FCP and do that, but I doubt that’s the issue here.
Did you delete your render files? Perhaps something is fouled up in one of the old render files.
If there’s nothing special about the clip that gives you the message, then why does it have to render?
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G5 Dual 2Ghz <> 4GB RAM <> FCP 5.02 <> Kona 2
ATTO 42XS <> Huge Systems 4105 Fibre