And this is FinalCut Express forum !
But I guess that Your Main / Start-Up / Boot / Mac OS Hard disk is filling up ?
You need more than 25GB free space on this ONE and may be 4-5 times more if You are working with HD-Material.
Even though You use two more external hard disks for "Main video" and "Preview files" the space need to be on Start-Up Hard Disk for Mac OS and application .temp files - as they can not be addressed elsewhere !
Yours Guessing - Bengt W (I use both FCE (4.0.x) and FCP (6.0.6))