I have a Samsung external USB hard drive, connected to a Windows 7 laptop. The drive contains films and photos. All was well, until I added one high-level folder to Picasa to view the photos. It imported nothing, and since then, Windows will not see the film files. Moreover, for the photos, Windows will not open them, and says "Windows photo viewer can not open this picture, because either the picture is deleted, or it's in a location that is not available"
If I plug the USB drive into a linux machine, all is well. The files are there, and I can open them.