Building mosaic failed

Hi

Using 10.7.1 I am having trouble with the mosaic option of the screen saver. I receive the error "Building mosaic failed," when I activate this screen saver.

When in system preferences, I see a simulation of the mosaic forming in the 'preview' window.

I am pointing the screen saver to my iphoto library. The other screen saver functions work. I tried pointing mosaic to other sub-libraries in iphoto like events and have the same problem.

I do keep my iphoto library on an external HD.

Any ideas?

AS