I have various files exported out of FCPX that open in Quicktime, FCP, and Davinci Resolve. The file types I've tried are HD ProRes 422 .mov, XDCAM422 .mov, and H264.mov. When I try to import in PP I get an error message saying We were unable to open the file on the disk. Several colleagues are having the same problem.
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This is a user-to-user forum, with volunteers (like me) that put in a lot of time helping our peers. There is oversight by the product support staffers.
That would look to be a permissions issue is my guess. That appears to be on an external drive connected via Thunderbolt, so you would need to check in the OS that Premiere has permissions to access that drive.