This is an old thread, but apparently Apple hasn’t addressed the issue. While researching a similar problem, I found a fix thanks to a discussion on Apple’s site:
The best way I’ve found to share a project is to make sure the file directory structure and drive name match exactly on both systems. That way there are no reconnect issues at all. Cloning the drive is the easiest way to do it and then simply rename the drive to match the original drive. I usually add an x to the end of the drive name so that there are no problems caused by having to identical drives on the same system. And then I remove the x from the drive name when I connect the drive to the other system (but before I launch fcp).
This instantly fixed the problem for me. I didn’t even clone the drive – I just changed the drive’s name. Not sure why FCP thinks the rate has changed when all I did was copy the file…!