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