iMovie project won't optimize for speed adjustment

I uploaded a large amount of QuickTime footage (about 7 hours worth) to iMovie and I'm attempting to speed it up (4x). Each time I try, it prompts me that the footage must be optimized before the speed can be adjusted...when I click OK, the progress bar appears and it begins supposedly optimizing. Several minutes later I get an error message saying "the clips couldn't be optimized. Error -50." I really need to finish editing this video and I've tried multiple times, even optimizing separately and trying to optimize/adjust the speed in fragments, using smaller clips of the entire thing, still the same problem. What would be causing this? Anything would help...thank you!