Hi Filmyak,
The error with adding sample data for asset r20.7 looks like the media cannot be read by AVFoundation. We would need to look at your fcpxmld to see what clip is actually referenced as r20.7. Alternatively if you can find r20.7 and transcode this to an AVFoundation friendly codec (prores). That should be that.
Regarding the drives, it will be an operating system issue, we sit one side of it, and the drive on the other. Are you able to use these fat drives with FCP ?
Yours Sincerely
James.