Hi Joe!
It’s a USB 3-connected HFS+ drive with externally managed media (consolidated into the “Final Cut Optimized/Original/Proxy folders but not as part of the library). The Libraries themselves are on APFS drive, either internal or external. Library A was created with FCPX 10.4.3 and updated to 10.4.4 as it came out, running on MacOS 10.14.2, 2015 rMBP and 2015 Retina iMac. Drives are fine. I did not notice that it would manifest after a crash. There’s all kinds of footage but only XAVC 50p and iPhone clips ever had this issue.
I did not manually modify links in the Library. I did want to, as I had an additional issue with this Lib before (several clips from Sony FS7 (XAVC Long GOP) were reported as Missing File and there was NO way of reconnecting or re-adding. Re-linking went fine but clips were still Missing afterwards, deleting from Browser and dragging back into it from Finder did literally nothing, re-importing from resulted in Missing files also). But no, I didn ‘t modify the Library manually in any way.
The black clips are created exclusively when creating a new library and dragging existing clips/projects into it. It doesn’t happen with all Libraries and/or Projects. I had this happen before, usually with Synced clips (with those, usually audio plays but video is completely black and there’s nothing that can be done about it other than manually opening the clip in timeline and dragging the video clip into it from browser).
When I re-tried creating the new Lib and dragging to it from old Lib, it was always the same clips that had the issue. There is no craziness in file/clip names, nothing that would differentiate them from the clips that passed without issues.
I can’t check your other suggestions as I don’t have the media drive with me. Thanks!