ALSO, be aware that El Capitan and iMovie v9 are NOT compatible. Problems with photos (they will NOT import) and video skimming us upside down.
For people who want to edit movies in the Mac environment the best course of action is to use an older operating system on your Mac which is quite easy to do.
If you want to do movie editing, and remain in the Apple club, (and I do) you've really got to keep a partition on your hard drive with OS 10.9.5. That way, you can run iMovie HD 06 or iMovie 09 (and set chapter markers with just one click) as well as iDVD, and use Disk Utilities to burn DVDs (or better yet, the new M-DISCs) with ZERO issues. Using iMovie HD 06 and iDVD I can easily edit a one-hour movie, set my chapter markers, and burn a Hollywood quality M-DISC DVD in less than 30 minutes. (M-DISC will last 1000 years.)
In fact, our organization has gone so far as to set up Mac Minis running 10.9.5, iMovie HD 06, and iDVD for the sole purpose of our movie editing. Basically, these machines are, "frozen in time". Someday, hopefully, in the future, Apple will provide better tools as they once did in the past. Until that time, this is our solution.