i don't think you can mount the image and then write on the image like that you need to patch the entire iso image with out mounting it or extract the files to a diferent folder if you have any more question just read the PS2 On-Line FiX tutorial it will tell you how to do a proper patch to the image or the files . . .
First time using the disc id inserter and when i finish entering in the disc id, i get a runtime error code '75' Patch/File access error.
Im setting up the .img file in nero imagedrive.
As alucard said, you cannot patch the DNAS file(s) mounted in the virtual disc. The files of a virtual disc are read only just like if they were on a real disc. You will have to copy the DNAS file(s) to a temporary location on your PC HDD, patch the temporary DNAS file(s), then reinsert the patched file(s) back into the ISO image.