Aperture adjustments to files on external drive?? How?

Hi all,

I recently bought a macbook pro and use Aperture a lot. Initially I had a Imac G5 which had a 400GB harddisk, sufficient to store my pictures. The macbook pro has only 100GB so I need to store the Aperture pictures on my Lacie external drive. Is it possible to leave the pictures on my Lacie when working which Aperture and make the adjustments while the files are on the Lacie? Anyone knows how to do that? Will I have a big speed loss by working this way?

Cheers,

Kim