After exporting an Image I wanted to go back into the development module and make another tonal correction. After clicking on develop I received this error "File appears to be unsupported or damaged". What happened? I had just toned and exported it. The file is on an external hard drive and is a Canon MKII CR2. I'm using Lightroom 4 on a Mac Book Pro. Thanks.
Did you tell LR to add the exported photo to the library upon export and are now by mistake trying to work on the "re-imported export"?