Corrupt files in Painter IX?

[Double-L]

I do storyboards for animation in Painter IX.5. I end up with eachpage being a multi-level document that I save as a .psd file in case I have to do some little adjustments in Photoshop. My problem is that occasionally, for no consistent reason that I can see, some of my files get corrupted. When I try to reopen them, I get the message "Photoshop compression method not supported. " This happens exclusively to files that I have NOT opened in Photoshop. And I can't open the corrupted files in Photoshop either. This costs me a lot of extra work and time. Is there any way to retrieve those files?

I'm using a Mac G4 mirror door tower w/ 1 GB RAM, with OS 10.4.8