Importing to external drive in Aperture 3.2, low disk space error for internal drive?

Hi,

Never had a problem like this before, hoping it's a quick fix:

I'm importing about 20G of images into Aperture from a CF card. The files are to be stored on an external drive. That drive has nearly 250G free. My internal drive has about 6 gigs free. (Which I assume shouldn't matter much, as it's only being used for thumbnails, metadata, etc. for these images.)

However, when I try and import, I get an "Insufficient Disk Space Error" that says I need at least 10G more on my "Aperture Library Volume" to do the import. Am I missing something? I have 250G free on the drive where the RAW masters are being stored...why would my internal drive need 16G (the 6 I already have, plus 10 more) for these photos?

Thanks so much. I have a deadline on these photos for tomorrow morning, and I really cannot afford to wait too long to solve this.

Regards,

David