Lightroom - Switching external hard drives without losing edit history

Re-importing photos is almost never a good thing to do, as you have found out.

Instructions for your move are here: https://www.lightroomqueen.com/move-photos-another-hard-drive-leaving-catalog/

What you’re asking for is reasonable and possible. The problem you ran into is that history steps and some other types of metadata, such as collections, smart collections, and slide shows, exist at the catalog level, so they’re not stored with any individual images. They’re in the catalog. Therefore, importing the same images to a new or different catalog does not bring any of that catalog-level metadata…it was left behind in the old catalog.

A lot of people have to move a Lightroom Classic setup from one computer to another, or one drive to another, so there is a “best practice” way to do it. After you copy the files to a new drive or computer, you open the same catalog you were using before, and tell it where the images moved to. You can do this by right-clicking* a topmost folder in the Folders panel, and choosing Update Folder Location. That will relink not only the images in that folder, but in all subfolders. And it will relink all images to their history states and other catalog-level metadata in that catalog, such as which collections they were in.

If you need step-by-step instructions for that, they’re in an article on the excellent Lightroom Queen website:

https://www.lightroomqueen.com/how-move-lightroom-to-new-computer/

*If right-click isn’t enabled on your Mac, Control-click the topmost parent folder instead.