After freaking out and tried every single possibility to fix this I realised that the problem was that FCPX has problems to handle and render .m4u files to preview of show this files on import window, was a shame because its a tipe of file that Apple devices creates.
So, I finally solve changing the extension of the .m4u files on the folders to .mp3.
Works so fine. I tested many times surfing in folders with those files changed to .mp3. But once I open one folder with an .m4u file... #crash
I couldn't surrender to format and reinstall everything from 0.
Hope this help you guys.