If you're on docker:

Could be that the path to the mounted disk isn't exposed. (Test: Go into docker container with bash and try listing the movies and try doing a cat textfile.txt to check read access)

Could be that even though the path is exposed the media is not writable. (Test: Go into docker container and do a touch command and see if file pops up). Not sure if relevant but is relevant for Sonarr.

Not mounted?

It could have been mounted once but is not anymore. Not sure how Sparky works but on Fedora Bazzite I had to add fstab line to auto mount the drive on power up.

Generally the rule is that if Jellyfin can see and read the files it can play them and will auto add if the path is configured.

It might be an ownership or access issue. Try explicitly enabling read/write access to the drive with chmod.

Sounds like the server is online before the drive is mounted?
Depends on how you connected it.

Anyway: Welcome to the party and happy collecting :)

If your media stack is containerized with docker (no idea how you installed it), you need to go into your docker compose file and grant the container access to your drive.

Upgrade to docker when you can. It's not as hard as you think. And it's an extra skill. If you can handle Linux you can handle docker

run it through docker? Give the path through /mnt or /media.

Are you on Windows or Linux? If Linux, is jellyfin installed on docker or from your package manager? The drive is mounted right?