Windows 7 IE9 ReadyNAS Security Certificate

Here is the process in which I have found works with Internet Explorer
- Browse to the site whose certificate you want to trust.
- When told "There is a problem with this website's security certificate.", choose "Continue to this website (not recommended)."
- Select Tools->Internet Options.
- Select Security->Trusted sites->Sites.
- Confirm the URL matches, and click "Add" then "Close".
- Close the "Internet Options" dialog box with either "OK" or "Cancel".
- Refresh the current page.
- When told "There is a problem with this website's security certificate.", choose "Continue to this website (not recommended)."
- Click on "Certificate Error" at the right of the address bar and select "View certificates".
- Click on "Install Certificate...", then in the wizard, click "Next".
- On the next page select "Place all certificates in the following store".
- Click "Browse", select "Trusted Root Certification Authorities", and click "OK".
- Back in the wizard, click "Next", then "Finish".
- If you get a "Security Warning" message box, click "Yes".
- Dismiss the message box with "OK".
- Select Tools->Internet Options.
- Select Security->Trusted sites->Sites.
- Select the URL you just added, click "Remove", then "Close".
- Now shut down all running instances of IE, and start up IE again.
- The site's certificate should now be trusted.

Give it a try and let me know if it works for you too.