Server error

Hi All,

I found an error when copying data server to external harddisk on Win 2003 Server R2. The error message is "Cannot copy XXXXXX:Not enough server storage is available to process this command.". Some files could be copied successfully. The error message occurred in the end  of the copying operation. The diskspace on the Server and harddisk should be more than enough. Any idea of the possible cuases? Thanks in advance.