FCPX media missing from timeline but is in browser and plays from browser.

In two recent projects using the same media around 1/4 of the clips in the time line are reported as missing showing the red and yellow warning notification. When I select Reveal In Browser the clips are there.

When I play the clip in the browser it's fine. If I then select the browser clip and then right click to reveal in Finder a window opens and correctly shows the file in the Original Media folder and it can be played from there.

The project is set up to use Original optimised Media not Proxy and so it's not a case of missing Proxy files. The project has always used original mai from day one never Proxy.

I am trying to export a video of the project for a client who needs it today but at the moment the only solution I can see is to re-edit the missing sections which will take me all day.

Any ideas how the timeline can be updated to show the "missing" media? This is not a case of re-linking as the files are already properly linked in the browser.

OSX 11.5.2 FCPX 10.5.4 All projects and media are on an external 7200 RPM G-Tech desktop hard drive which is my usual editing drive.