My notes on this.
Error -49 opWrErr File already open with write permission
Trash the preference files while application is NOT Running.
from Karsten Schlüter
Some users notice on exporting larger projects from within iMovie that this operation is aborted with an 'error -49'
This issue occours only on MacOs machines using 10.7x
try switching-off the Local Mobile Backup
in Terminal copy/paste
sudo tmutil disablelocal
Yours Bengt W