First off you'll see something like this in FCP 6/7 if you hover over the render indicator in the Timeline:
|Actually, the solution is mentioned right there.|
If you read the entire message you'll see you need to look here:
Where it reads "Thumbnail Cache" just bump up the Disk and or RAM setting a bit to cover the amount of images you're using in the current project to eliminate the warning, and help FCP a little bit with rendering all those stills.
I've left mine bumped up over the years as projects have caused the warning and left it with no ill effects. You'd think this would be dynamic after all this time but then again, OS 9 had separate application memory allotment settings in the old Get Info window.