Most likely the volume is either damaged and needs repaired using Disk Utility First Aid, or the volume is Filevaulted and needs to be unlocked. It is also possible there is an issue with the drive or the optical drive bay cable/controller.
Within Disk Utility click on "View" and select "Show All Devices". Select the physical drive and try to repair the drive using First Aid.
Try connecting the drive externally to see if it will mount as this will tell you if there is an issue with the optical bay's SATA controller or cable.