why is aperture filling up my local mac disc when exporting a video to external drive?

I`m exporting a 1 hour slide show from aperture with music.

I`ve outputted a 720p ipad version (done), now doing a 1080p HD version export.

It gets to about 65% (twice) then gives me an error that the disc is full. I had 45 gig left prior to exporting on my laptop retina.

why is aperture filling up my local disc when i`m exporting to a Lacie thunderbolt drive?

I have zero disc space left, I cannot even perform a screen capture or do rudimentary things on my mac, and I cannot find the video rendered (data exported) so far that has clogged up my system to full... where is it being stored? what do I need to delete to get this space back?

please help urgent, i need to get this wedding slideshow out by tonight and move on to other editing tasks.