Q: Why does FCPX have to re-render my clips?

I am using the latest version of FCPX  (10.2.1). I started noticing this issue either when I updated to one of the recent versions or when I updated to Yosemite, I can't remember.

I am editing on my new Macbook Pro and the project Library is located on an external drive. Every time I close FCP and re-open it, every clip that has an effect applied (which is most of them) shows that it needs to be rendered again. The storage locations for my media and cache are set to "in Library".

This is also happening when I edit on my Mac Pro ( same FCP version, same storage settings) while editing from an external drive.

If I am editing a Library that is located on my main boot drive, there is no re-rendering issue.

This is a new phenomenon. I have been using FCPX for long time and this only started happening within the past couple of months.

Any ideas on what is going on?