We have a very extensive image library and we use external drives to store everything due to their size. The entire catalog was on one external drive which has been retired. LR is now showing the majority of these images as "Missing". When I try to relocate them from the new drive it tells me that they are already in the Catalog. However, LR is not relinking these images, just telling me they are already present. This isn't helping at all! Is there a way to change all of these to the new external drive?
Sounds like your previous attempts were to import the images again (always a bad idea) when you really wanted to "relocate" the images.
Since you are looking to relocate missing folders, all your work needs to be done in the Library's Folder panel.
I suspect you might have another image with same file name, so If you are sure that the file you are linking the missing file is correct or match with the preview.Then please ignore the message and re-connect your images.
Let us know if it helps.