You should be able to drag your movie from your desktop into an iMovie timeline. Is that the way you are doing it? Also, I wonder if there is either a glitch at the 6:40 mark or you don't have enough free space or application space to complete the import. Possibly also there is a corrupt preference in iMovie that needs deleting and resetting. Or maybe an update of iMovie is in order. Nothing to do but try some diagnostic procedures.
First, check your available space and your import procedure. Then scroll through the movie in QuickTimePlayer and look for black frames, white flashes, artifacts, and any other signs of corruption. Cut out any that you find. Pay particular attention at the 6:40 mark. Save the movie and try again to import into iMovie.
If still probems, try creating a test movie and see if that one has import issues.
If still problems, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can import the full movie.
If still no luck, another thing you can try is to import your movie into a newly created project and see if that cures the problem.
If still no luck, try opening iMovie in a newly created library and importing into a project created in the new library. To do that, open iMovie while holding down the option key and select to create a new library in the dialogue box that appears.
Failing that, update iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from
there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.
-- Rich