Not exactly your situation, but I can’t see the contents of private folder in finder.
I can open the private folder with finder, but then have to right click and “show package contents” on the AVCHD folder, then repeat for the BDMV, then open the stream folder to get to the mts files.
Not sure what the issue is that causes this. Someone on the apple FCPX forums suggested it might be a misformed or hidden name in the file directory that ML/FCPX doesn’t like.
-Alex-