I'm using a rendered After Effects file in an FCP project - alas, I made the mistake to delete this file. I re-rendered the file again, made sure to give it the same name, and now try to replace the 'missing' file with the new one. Here's the trouble: right-clicking on the red missing file in my used media list I ask to 'reveal' the file. This opens the window 'original media' with a bunch of 'alias' files. Right-clicking the alias file in question to 'show origin al file' gives me the option to 'fix alias' - selecting then my newly rendered replacement though gives me a window with 'unexpected error occurred (error code -36).
Just relink the file to the new file. Select the file in the timeline or browser and go to file>relink files.
If you rendered it to the same location that it was previously, with same name and same exact specs, then FCPX will see it automatically. That is assuming you were using leave file in place, which you were because there was an alias in your original media folder.
Oh my - I made it, thank you so much for your help! I went to 'Files > Relink Files' and tried to select the correct file, alas FCP crashed. Opening it again. though it did relink and all is good again ;-)