"The patchers are only for installed applications. They can't be used to update the floppy disks. Make a Startup Disk from the disk image, trash the Disk Doctor on it, and copy the updated Norton Disk Doctor 3.5.1 from your hard drive to it. Change its name to "Disk Doctor". That is critical. It can't be "Norton Disk Doctor" (like it is on the version installed on the hard drive)."
Note: This may only work with PPC version of Norton Disk Doctor (if that). The 68K version appears to big to fit on a system floppy any more.