Here are my 2 cents. Your dad's photos are an amazing legacy for your family. I would merge the catalogs. I would first use your dad's computer to consolidate all your dad's images from the existing drives onto a new larger drive, then create a backup of that new drive. If necessary, build a simple unified folder structure from the top down, by moving the scattered folders from the original drives into a new logical order. When you merge the catalogs, you can keep the new drive with your dad's images independent from your drive. You can then create and use Collections where both your images and his can co-exist. From here, you can build slideshows, books, or online galleries to share the images.
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