Disque SSD externe pour travail et sauvegarde

Copy (not move) the photos in your operating system to the external SSD, then re-connect in Lightroom Classic, then when you are sure everything is working, you can delete the photos from the original drive. Follow the instructions in "Part 2 — Updating Folder Location" here: https://www.computer-darkroom.com/lr2_find_folder/find-folder.htm

But, your title worries me ... if you move the photos to an external SSD, then this same external SSD cannot be used for backup (a different SSD could be used as backup)