Photo displays in lower grid, once I select to view and or edit part or all of the photo goes gray. This is happing to JPG images, when I look at the image in "Preview" on the iMac it is the same. It appears LR has corrupted or changed the original image.
The image has changed while viewing in LR, it may have been corrupted but it was only while being viewed in LR. I don't think this would make a difference but the image is being stored on an external drive connected by a network connection. I'm worried this is the cause and I might damage old photos that are on this drive also when I open them.
When you say open them, do you mean when you view them? Open the folder where the photos are stored and check the modified date. They should still have the same date that you took the shots. View a photo and see if the modified date changes? If it doesn't, that means that Lightroom is doing nothing to them. Is it possibly the graphics card that you are using? Try turning that off in preferences.