One of my user accounts had this problem on install. I
think FCP 5 has a bug where if you choose a non-default scratch disk upon first launch, it freaks a bit. I could be wrong but...
The solution for me was to delete the preferences, and not change the default scratch disk in the initial boot screen. I then changed it inside from System Settings. All has since been fine.
Try that and let us know if it helped.
- Justin Barham -