Yes, as I said they appeaerd in Irfanview Thumbnails, and opened in Irfanview. And it's not a drive problem because newer images on the disk, last year for instance, open normally. I've discovered another curious thing, if I copy an image to the C drive I can see it in Bridge, and open it in ACR, everything works normally. I suppose I could copy the whole thing, folders and sub folders, and then copy them back again.
Wait, wait, I just copied the image back to the USB drive, original folder, and guess what? Everything is normal, Bridge can see all the images. It'll have to go down as "just one of those things."
Thanks again for your response.