IMovie error -50

Not sure if you've found the solution or not, but discovered from this link: http://rockonflash.wordpress.com/2011/04/28/imovie-eating-memory-on-ipad/ indicated that if you make a change to the original project it frees up your space.

Additionally, I've read that it's better to save/export the project to iTunes instead of locally, and then distribute to where you need it.

Hope this helps you, or any other poor traveler who runs into this issue.

This is error -49, OpWrErr: file already open with write permission.

Turns out, this is related to Time Machine running at some point while the video is rendering.

It's possible to address this in one of two ways:

1. Disable MobileBackups.
2. Disable Time Machine entirely.

I chose the latter of the two and the export succeeded fully.