I bet that when you look under your scratch disk settings, it says you only have 2GB left (not the finder which is correct). This is a FCP bug.
It happened to me a couple of times. Most of the time it was right after I used Automatic Duck to Export a sequence.
Did you try relaunching FCP? Sometimes this basic move helps.
Other times it's a corrupt render file. Look in your Render Files folder and see if there is one file that is suspiciously large, by suspiciious I mean inconsistently large when compared to the others. Delete that one file and rerender.
Good luck.