I have started seeing this periodically since a recent update, probably 6.0.4 but may have been one of the Pro-apps updates.
What’s puzzling me is that the clips it chooses appear to be random. For instance, after a full day of digitizing, it notifies me the next day that one of the dozens of clips I digitized yesterday is “not optimized.” As far as I can tell, it is identical to every other clip that was digitized including it’s media drive location. I’ve used Media Manager to copy it, as suggested. Still pops up.
As it happens, this one is a small, inconsequential clip.