External Hard Drive unable to open files

Hello,
This morning I started to transfer some new pictures to my Western Digital 500g usb drive. later I tried to open the files but they wont open. I can see the files in explorer and it lists properties on them but if I try to rename or move them it says can not rename<filename>: cannot find specified file. Make sure you specify correct path and file name. All the files are likethis not just the ones I added today but also the ones that have been working. any ideas.