trying to download firmware upgrade

At what point does the message appear about the disc being corrupted?

There are several steps in doing a manual firmware update (loading a previous version is still an "update", ha)

• Navigate to the Netgear web page with the desired firmware for the product.
  https://www.netgear.com/support/product/RBR20.aspx#download 
  If you are have previously upgraded to a version that you are not happy with and want to "go back",
  click on the "View Previous Versions" to find the firmware you want.
• Click on and download the firmware files for both router and satellite.
• Extract (unzip) the ".img" file for both the router and satellite from the respective .zip files.
• Update the satellite first.
• --- Navigate to the Satellite web page.  The IP address of the satellite should appear on the list of "Attached Devices"
• --- Log in with the usual credentials, "admin" and the administrative password
• --- Select the bottom menu item, "Firmware Update"
• --- When the web page asked you to Browse to and select the firmware file, navigate to wherever you extracted the .imp file
• --- The process should complete
• If there are more statellites, do the same thing on each.
• Finally, update the router
• --- Navigate to the Router web page
• --- Log in with the usual credentials, "admin" and the administrative password
• --- Click on the Advanced Tab
• --- Click on the menu Administration
• --- Click on the bottom menu item, Firmware Update
• --- On the Firmware Update page, click on the Manual Update tab
• --- Click on the button that asks you to Browse and select the firmware file
• --- Find the router .img file and click on it
• The process should finish and the router will reboot

So, at which step does the error message appear?