Final Cut X User Folder on Different Drive

I have an 64GB SSD boot drive in my 2009 Mac Pro with 20GB free and a 640GB HDD for user directories. This seems to work well except when I try to transcode/optimize files in Final Cut X I either get a notice about "not enough space" or it just sits at 0-2% completion with no processes running for an eternity. The thing is, FCPX doesn't seem to be trying to fill up my SSD it just seems confused.

Is this the case? Do I need a bigger/emptier SSD just to appease the X gods? I seem to find contradictory info on this when I google.