It is apparently a Mac issue. A colleague of mine had a similar if not the same problem trying to migrate his photo library from the older OS's iPhoto to El Capitan's Photos on a brand-new MacBook Pro. (I'll leave the description of his frustrated fury aside.)
The solution (once he became aware of it) was to open Photos, select Import (Library), find Library under iPhoto and click on it. It worked lickety-split.
The same was true for iMovie. El Capitan apparently does not come with Movie, but he was able to download it for free (presumably from the Apple Store). And then he proceeded in the same fashion to migrate his iMovie files to Movie.
Give 'er a shot.
