If you can not copy the files even in an empty folder under "My Documents", the reason can indeed be a corruption on the source drive. This can happen with USB sticks. The files are then not accessible anymore even though they appear in the directory.

I have not yet had such a case, but there may be solutions on the web, google for "usb stick file recovery".

Otherwise do a fresh A7 installation from the A7 version on the download page.