Guys I'd like to also confess that I too have the same problem as Dirk.
I work of a 2011 MacBook Pro (running 10.6.8) with a CalDigit VR attached, if I Quit FCP X, eject the CalDigit, Power everything down, then power everything back up again and open my last project (which was totally rendered), everything needs to be re-rendered again.
For the time been what I've been doing is splitting my project in section, after completing/rendering, I export the storyline to a master ProRes 422 QuickTime file.
I've tried all the obvious things, trashing prefs, deleting renders, HD permissions etc... I found non of those to make a diferance.
Dirk, I strongly suggest if you haven't finished your edit, leave your system running till you do, then export to a ProRes.
I know it's annyoyng, but we really don't have a choice right now.