You can tell that you need to relink files when there is an exclamation point over your Event and the files show up as scary looking red windows.
To relink media:
1. Right click on the Event in your Event library and select: Relink Event Files.
2. A dialog box will appear listing the files you are missing. Click on the first file and click: Locate All.
3. A finder window will appear – navigate to the folder where you stored your original media.
4. Select the file within the folder that FCPX is looking for – you can look at the bottom of your finder window if you forget which file name you are searching for.
5. Click ‘Choose’.
6. FCPX will locate all of the files within the folder that it is looking for and you will return to the dialog box. There may still be some missing files, if so, repeat the process until all of the files have been located.
7. Click: Relink files, and FCPX will relink your media.