Where exactly are you looking?

In Disks, in Files, or in Plex?

The GNOME Disks app will tell you if it sees the physical disks and partitions, and let you set up mounting options. Hit the Mount button (Play icon) to mount the partition once, or hit the Config button (Gear icon) to configure automatic mounting. For media disks, you probably want to mount at startup, and I’d recommend setting the “ldentify As” field to “Label” (which will also give it a “friendly” path like /var/mnt/Media).

In GNOME Nautilus/Files, you may see the disks, but unless they’ve been set to automount, there are lots of reasons why it may have failed (most commonly the dirty bit was set on the disk due to unclean dismount, fix with fsck command).

Finally, in Plex or other apps (notably Steam, if you have a separate Steam games partition), there’s a design choice in Bazzite/Atomic Linux distributions to use Flatpaks for most applications. Flatpaks sandbox the app from most of the rest of the OS, including disk access. This is intentional, but annoying if you do not know about it. Use another app called Flatseal to modify the permissions on the app’s Flatpak to allow it to access the other disk under “File System”, as granularly as it makes sense, (or just all disks if you’re a chaos demon). You can also do this using the flatpak override command, but you need to know the application identifier.