i am on a Macbook air with yosemite 10.10.5 and my video exports fail.
Trying to export it thru Photos menu - select file > export and choose my to LaCie share external HD but got that error.
...im able to drag or copy the file to my desktop and from there move it to the external HD but can't do it directly, don't know why. tried to export it to my internal HD but didn't let me either. already looked into this post How to move all iPhoto video files to an external disk or a NAS?