Maybe this is a known issue...but I was having compressor 3.5 issues today and took me a minute to figure out that it was because the install of FCPX had moved the entire suite into a folder...which prevented the app from updating to 3.5.3
I'm guessing this is why you create another partition for your FCPX install! :)
You can move the FCS3 programs back into the application folder and throw the FCPX programs into a folder labeled FCPX or something. Apple has a tech note describing this. Updating should work after that.
bizarre that they didn't imagine people using 2 versions of their software at the same time? Especially since the new one can't open the old one... ugh... this whole thing has me confused.
I did a quick search and found a helpful hint. When I tried to move the contents of the newly created folder with my FCS3 inside...it would only copy, I couldn't move them out. Found a blog referring to changing permissions and unlocking that folder to modify. I had to copy, and now I have 2 copies...now I know how to get rid of the other copy! :)