My next try at finding out what was happening would be to check the location of the files. A root folder of a drive can only contain so many files, (depending on the file system used, like Fat32 or NTFS). I would count the number of successful files in the root, and see if you are over that, based on the File System restraints. When you rightclick a drive, and Properties, it shows the file system in use for that drive.
Errors Creating Files or Folders in the Root Directory:
Rating posts helps other usersMark L. Ferguson MS-MVP