Hey. I have the same problem. I installed Final Cut Studio on a brand new erased, zeroed install of Leopard. 10.5.1 update downloaded and installed manually from the finder. Software update ran all the updates but said an error occurred and Motion 2.1.2 did not install "Try installing from the finder". I downloaded and tried a manual install but all drives were blocked out with the warning sign, including the boot drive.
This had worked fine on another partition a few months ago I ran as a test to make sure everything worked fine before updating my main drive. Everything is new and fresh. No mods. No hacks. No Pirated software. I solved it by deleting the Motion App from my new install and replacing it with the Motion App (which had previously successfully updated to 2.1.2 from the old Leopard install.
2.66 x 4 Intel Mac Pro, 4G RAM, 160G System 500G Scratch, Media, 320G Photo Capture, 320G Video Capture. 733 Quicksilver 1G Ram 120G system 60G backup 160G and 200G capture.