LRCC, my old external hard drive is full.

There is no reason for you to be scared out of your mind about losing images. If you are adding a new external hard drive you have two choices. If it's a bigger hard drive that will replace the old one then all you have to do is copy the images from the old external hard drive to the new one and plug the new external hard drive into the computer. Ensure that the computer identifies the new hard drive in the same fashion that it did the old one, and continue on.

If you are adding an additional external hard drive then just add it. It will be treated as a new additional drive, identified differently. The old external hard drive will still be accessible that has the 200,000 images on it, and you can access them as you have in the past. All that's necessary is for you to take a little time to think about what you are doing and where your images are. It's a simple matter of image management. Lightroom is not some mysterious monster that destroys images. All that is necessary is for you the user to think about what you are doing.

There is no LRCC. There is Lightroom Classic, and there is Lightroom, which stores images in the cloud. Which version are you referring to? What is your operating system? Please provide details about your computer system.