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Your present approach is correct. Ensure all these pages are tagged as noindex for now. Remove the block from robots.txt and let Google and Bing crawl these pages. I would suggest waiting until you are confident all the pages were removed from Google's index, then check Yahoo and Bing. If you decide that robots.txt is the best decision for your company, then you can replace the disallows after confirming your site is no longer affected by these pages. I would also suggest that, going forward, you ensure any new pages on your site that you do not wish to index always include the appropriate meta tag. If this issue happens again then you will have a layer of protection in place.