Are you trying to save to a flash drive? If "yes," check to see if it is formatted as FAT32; this format has a size limit on files -- 4gb I think. It will return an error message similar to yours, even though the drive may have a great deal more empty space. If this is the case, use the "convert" command on the command line to change the file system to exFat32 or NTFS.
Thanks for the replies fellows, I think I've cracked it. If I create 'new' and replace 'untitled' with my own name and save as everything seems to work. Brian