SYMPTOMS: FCPX 10.4.7 in macOS Catalina becomes completely unresponsive while stuck on rendering media in your timeline. 





TROUBLESHOOTING: Toggle off rendering and see if any of the media in your project is grayed out or "missing" over the dotted lines where rendering stalled.  You can also confirm the media file is gray in the FCPX timeline, FCPX browser, and the Catalina Finder location. 





CAUSE: If Quicktime can't play it, or it only plays in gray, it's now incompatible on Catalina.  Right-clicking the file and looking at its Get Info in Finder will tell you the incompatible codec the video or other media was created with.  The list is here: https://support.apple.com/en-us/HT209000.  It's probably incompatible media created with 32-bit codecs, often obtained from stock media sites or other such sources.  Hopefully, this only impacts incompatible media added recently by users, rather than for some strange reason FCPX failing to detect, prompt or convert the media over an unknown much longer period. 





FIX: You'll need to right-click and disable that media and/or convert it, or simply download and replace it with the 64-bit compatible version of the same media, usually for free if obtained from your paid subscription of a stock media site like Storyblocks.  Alternatively, a workaround for many files might be to open the library on a computer that still has Mojave and 10.4.6 and make sure it detects and converts all incompatible media it finds before upgrading to Catalina. 





CONSIDERATIONS: FCPX 10.4.6 or 10.4.7 may have not gotten the chance or failed to detect and convert some or all of the incompatible media files in Mojave.  Before I upgraded to macOS Catalina, I always allowed 10.4.6 to do this because the prompts made sure all of us knew this was coming.  But perhaps what happened is that recent incompatible media was added to FCPX 10.4.6 and then the application was closed before getting the prompt.  Also, FCPX 10.4.7 was released independently from Catalina.  So there are a few potential windows where incompatible media might have worked between upgrading from FCPX 10.4.6 to FCPX 10.4.7 and then upgrading to Catalina.  This is unconfirmed, but perhaps 10.4.7 on Mojave still supported the incompatible media but no longer prompted to convert it as that functionality was removed.  If so, the previously undetected incompatibility would not be realized until after upgrading to Catalina where there is no support in the operating system for the undetected 32-bit codecs used to create the now-incompatible media.  Perhaps there's a bug or upgrade workflow improvement that Apple can consider adding in future updates.