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.
I'm guessing it's using the boot drive [its own Library structure, in fact] as a scratch disk &amp; making the video as raw data, which it will then compress &amp; copy to the desired location. 

You cannot dictate which drive it will use for scratch, unlike Photoshop, so the only thing I can think to 'fix' it [other than start deleting stuff] would be to move the Aperture Library to a larger drive. Double-clicking the 'new' library will launch Aperture with that as its new default.

You could always move it back afterwards.