Could not save as "image.tif" because an unexpected end-of-file was encountered.To keep Xsan and Photoshop happy together, go to Photoshop's Preferences and choose File Handling. Under File Saving Options, select Image Previews and then choose Never Save.
I believe the error occurs because the Xsan filesystem just doesn't like all the resource fork data that Photoshop saves with images when the image preview is turned on. I was absolutely pulling my hair out until I figured this out.
[robg adds: I can't verify either the problem or the solution for this one...]

