FH10 won't import CMYK files from Photoshop10

I've got a large file 24x36" Freehand 10 document, with very little lettering. I can import a RBG TIFF file created from a digital capture of artwork. It's a 300MB file. When I change the PS file to CMYK, Freehand error message says "this is an invalid file." That doesn't tell me anything. I've created the FH10 file at 50%, and it will be doubled in size for output, so I need this 300+size file.
I have an IMAC G5, 80GB hard drive, but only 3GB left. I usually like to keep 10% open space. But, this is darned ill luck for such a heavy deadline I"ve got and I really appreciate if anyone has any advice.
Thanks,
Linda