I never said anything about external drives. All of my video editing is ALWAYS on the SSD built into my MacBook Pro - no exceptions. I haven't memorized the exact formatting used - but I always pick the most typical - whatever that'd be. Nothing funky. Just normal stuff here. Nothing to see.
Eventually FCPX was able to share the complete video - after I first let the "Transcoding and Analysis" finish.
In the past I never noticed any issue with sharing a video while "Rendering" was in process. But, when I tried to share while Transcoding and Analysis was running FCPX (I'm using version 10) would only produce part of the video - with no errors whatsoever. FCPX acted like this is normal - yet only produced part of the video - not the complete video. Weird. Weird. Weird.
Slowly - but surely - macOS is behaving more and more like your typical Microsoft operating system. Sideways. And no one knows why.
I'm hoping it's just the fact that I'm using a 2015 MacBook Pro running Monterey. Hopefully I can buy a new MacBook Pro soonish.