Cleanplates not working

I’m working on an 8 processor mac with 17GB memory os x 10.5.7 and the footage is 168 frames of 16 bit tiffs at 1920x1080. I track an actor against a dark background, then go to do a remove in preparation for pulling a matte. When I remove with no cleanplate, everyting is fine, but the removal is a little blurry, as is to be expected, since the actor never completely clears frame. I created a cleanplate to use, but everytime I assign the clean plate I get the following error after about 24% of the frame is processed:

“Failed to transform image /Volumes/2TB_LaCieRAID/TTIHAY/cleanplate.tiff”

I’ve tried targas as well as tiffs, and I’ve tried various color depths on the cleanplates - nothing seems to work.

What’s the secret handshake?

Thanks in advance!