I imported clips and confirmed they were in fact in the event browser.
The next time I entered FCPX, approximately 50% of the event clips had the dreaded red/yellow "Missing File" icon. These clips were imported to a library project that was previously on a different hard drive. They were imported to the new hard drive after I changed the storage location of that particular library of events to the second external hard drive.
I've selected the clips that are missing. Choose File > Relink Files When the new window appears I cannot find a match. I click on the devices icon, even the hard drive that used to contain that library title, and I don't see the missing files. Makes no sense. I know for a fact the clips were at one time in the library. I never erase (initialize) a media card before I confirm that media has been imported.
If I'm missing something in this recovery process, please, please respond with possible solutions.