Does the user running the jellyfin service have permissions to read the folder/files? You aren’t running jellyfin in docker by chance, right?

So when you go to create the library in jellyfin you don’t see the external drive but see your other drives as options?

Did you create a library in jellyfin?

Can you browse to the files via the mount point? Is the external drive listed at all?

From there you want to make sure you have the location listed as the library location in jellyfin. Then make sure the naming/file structure is correct according to the jellyfin documentation.