There are several approaches.
One option is to export your catelog as a catelog to a new replacement drive, selecting all images and selecting the option to also copy the images. When that process is complete, then you can reimport that catelog. You should now have your images on both drives and your catelog will have all the correct metadata for images on both drives. I would first run the option in Lr to check for any missing files, so you can try and fix any anomalies before you start major file operations and avoid confusing any issues which may pre exist with the actual recovery process. Also I would make sure that you do not have the option to prevent duplicates set.
However, I do not understand how if you make changes on your production image that the changes are automatically applied to the backup images. Do you manually synch the settings.
If you do not know how to find the Lr feature to check for missing files, then Google it. I am posting this on an iPad and have not got Lr available to me right now to get the menu location. I will post more specific notes on this when I get to my office machine. The find missing images process creates a collection which contains a list of all missing files. With 135,000 images built up over time, there may be some surprises within your existing production data.
Try the export as catelog for say a small sample folder of images first, to be happy you have the mechanics and all the settings correct, or better still, do a dry run on a different machine with some test data.