Is there anything about your configuration you could tell us that might help? For starters, if you are dragging the files from a Finder window to the flash drive iTunes is not involved. You would have been better off posting in the operating system (OD) forum for the OS version you are running (your profile does not include that).
Error -43 is an OS "file not found" error. This seems to happen occcasionally when a second drive is involved, particularly one shared with a Windows machine. E.g., Archived - Mac OS X, Mac OS X Server: "Error -43" When Using Windows File Sharing (SMB) - http://support.apple.com/kb/TS2039 - "There are three causes for this: illegal characters, a permissions issue, and/or a non-existent share point."
Try using Disk Utility to repair permissions on the drive.
Drive corruption is possible, Use Disk Utility to repair your drive. How you do this exactly will depend upon your OS version.