render files lost - drives me nuts

I believe this is a topic well known to you FCP guys.

I have a number of projects that resides on one project disk.

As long as the disk is connected to the same computer, it all seems intact. But sure, every now and then I can close the project and when I re-open (the next day or so) my render files are lost.

And they are sure lost when I move the disk to another computer. (Sure, I have set the Scratch disk in System Settings, even prior to opening the project).

Do you guys have any idea why this is happening? It's quite tedious having to re-render all the time...

Any tips/suggestions/solutions are greatly appreciated!