need to move videos from iphoto to Lacie external hard drive 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?
You nailed the problem when you tried to export to your desktop instead of the external drive. It's the export process that's broken.

You could pursue two avenues:


Dig into the video format of one specific file and work through that here. 
Contact Apple support since your version of Photos is likely new enough to get support (or free support on twitter would get you the Apple KB that lists what video formats are supported for export.