This problem may be related to Roon not having permission to access the drive. Can you try this:
On the Mac open System Preferences > Security & Privacy, go to the Privacy tab and on the left select Full Disk Access. Now add Roon. You’ll need to unlock the padlock and press +.
Now do the same for Files and Folders and make sure Roon is added and Removable Volumes etc. are checked.
Restart Roon and try again.