"the plugin QuickTime power plug can't be replaced.
You have solved the problem of newer Vs older "Foreign File Access" ( a sub-extension of the Apple CD driver used to read ISO, High Sierra, PhotoCD, and other foreign file formats).
You have moved on -- to a clash in versions of QuickTime Extensions. The version of the "QuickTime Power Plug" extension is different from the one the Installer wants to Install. In addition, It is possible the extesnion on your Hard Drive is locked. To handle the error and Quit, it want to go back to diskette 2. If you can resolve the error, it is possible it will shut up about diskette 2.