This is useful function to know about if you are wanting to move a folder (or even your entire photo Library) from one drive to another drive. For example, say you are running out of space on one drive and want to move photos to a new drive to free up space on the original drive.You could use Lightroom to move the folders via drag and drop within the Folders panel, but I don’t recommend doing that when a large amount of date is at stake. Using a move command is risky, because if anything goes wrong in the middle of the transfer you could lose data. I prefer a technique that involves copying the folders to the new drive outside of Lightroom, updating Lightroom to this change, and then later manually deleting the files from the original location (outside of Lightroom).
Open Mac Finder / Windows Explorer (or any file browser or folder synchronization software of your choice).
Copy the entire folder structure (as-is without changing the folder structure or photo file names) from the original drive to the new drive.
Once the copy operation is complete, Expand the Folders panel and right-click (or however you trigger the contextual menu) the parent folder (of the folder structure you copied to the new drive) and choose Update Folder Location.
In the resulting dialog box that appears, navigate to and select that same folder in the new drive (the one you just copied over). Lightroom will update the catalog to point to the folder in the new location (and everything inside of that folder). If you have all your folders/photos in a single parent folder then you are done, but if there are additional folders at the same level as that top folder you just need to repeat steps 4 and 5 with those folders. Give it a test run to make sure everything is as it should be before removing the files from the original location.
Having a good backup in place before you do this is always a good idea. I use this function every time I migrate my photos to a new drive.