You didn’t mention that you set up the drive in Settings > Storage. Did you?
How much data is on there?
Was the drive attached in the past and did it work then (i.e. everything visible) or is this the first time?
Asking because if it’s several TB of stuff the Roon will need some time to import it. Are you seeing a spinning circle top right in the Roon window? Because that would mean it’s still importing.
If you go to Settings > Library > Skipped Files, are there any listed?