OK, I can’t get Roon to accept a separate drive as a source. I have a Macbook Pro, Catalina 10.15.2; I received Roon for Christmas, and it easily accessed my iTunes then. I just finished putting all my library on a separated ssd drive (new, lossless files, with no duplications, etc.). I dumped my existing iTunes, and the new library is too big for my Mac’s hard drive. So, I just want to access music on this separate drive. Won’t do it. Here are some screen shots of what I get…
I have moved your post to support, as it was not related to the NAS category.
Regarding your issue:
Access to external folders is blocked in macOS Catalina by default.
(@support maybe an info window like this would be good) .
I guess this is the issue in your case.
Try to quit Roon and open the macOS system preferences.
Navigate to “Security & Privacy” and click the “Privacy” tab.
You see a column “Full Disk Access” on the left side.
Click the lock at the bottom of the screen and enter your user credentials (it has to be an administrator account).
Then click the “+” button on the right side of the screen and add Roon (from your Applications folder) to the list.
Afterwards it should work.