I'm not sure I understand the question.
All I did was copy the folders, containing mainly FLAC files, from my Windows PC and pasted them into the SSD - and indeed the HDD. I have not consciously created a 'filesystem'.
Should I have done so? If so I may need a bit of help understanding exactly what is needed.
Just to clarify, you attached a drive to your windows box and copied the files, then moved the drive to your IdPi4.
When you do the Diskmenu, option 4, do you see the drive displayed there?
If so, you don't need to add a library, just add a auto-import entry in MC that points to a folder on that drive.
If you DON'T see the drive under option 4 on the disk menu after having chosing the Read-only or Read Write option for external disks and rebooting then there is another issue.