From time to time you may see that a tiff file is showing as corrupted or invalid.
The most common thing that will result in creating an invalid tiff file is the way you save your tiff from Photoshop. You must save your tiff as Interleaved and not Per Channel. When you save it as Per Channel, you will generate a tiff that Exposure cannot read.
Try re-saving the image as Interleaved and then try it in Exposure again.