I had the same problem, but found the solution:
The boot sector is defect (Recuva: Unable to read boot sector), you have to format the USB disk, but be sure you do a quick format, so the first sector will be restored and the data will be preserved.
Now you have (what Windows beleves) a clean disk, but Recuva (or other carving tools) can recover the data. Good luck!
In my situation, all files has been recovered. The USB disk folder structure and some file names were not preserved.