Some 3000 files of metadata lost

[windowsdan]

I've had an issue where JellyFin, running on a debian machine, uses 10 x SATA connected drives as it's content and what the library has been creating it's metadata from, perfectly fine until last night.

For some reason all the drives dismounted at around 3am and requested authorisation to re-mount. So I came back to the server this morning to find 10 x authorisation boxes and nearly a blank JellyFin library.

Unfortunately my JellyFin is set to do a library scan at 4am every night. Is there anyway to get back all the data I had as all the files are now scanned back into the library in completely the wrong order and recently added is completely wrong, and so is a lot of the content I had to manually override the metadata for

[Venson]

If you dont have a backup then no.
When scanning jellyfin removes everything except watch data from its database that is no longer present.

There are 2 ways to mitigate that in the future:
1. Backup. Often.
2. Use NFO/image saver to save metadata directly next the media
3. Just follow our naming scheme and folder structure so that you can always rescan media easily
https://jellyfin.org/docs/general/server/media/shows
https://jellyfin.org/docs/general/server/media/movies

[windowsdan]

If you dont have a backup then no.
When scanning jellyfin removes everything except watch data from its database that is no longer present.

There are 2 ways to mitigate that in the future:
1. Backup. Often.
2. Use NFO/image saver to save metadata directly next the media
3. Just follow our naming scheme and folder structure so that you can always rescan media easily
https://jellyfin.org/docs/general/server/media/shows
https://jellyfin.org/docs/general/server/media/movies

Thanks for your reply. I feared this might be the case.

I do already follow the correct naming convention for shows and movies (unfortunately certain ones still try to use the wrong metadata, no matter the metadata provider I use) so getting everything back isn't the biggest problem. It is all the recently added and time scanned into the library data that I want to maintain. 

What would need to be backed up to specifically keep that data?

[TheDreadPirate]

Depends on the OS and install method.

Windows - C:\ProgramData\Jellyfin
Linux - /etc/jellyfin, /var/lib/jellyfin, /var/cache/jellyfin (optional, but recommended)
Docker - /config (wherever you mounted /config on the host)