Thanks for reaching out with your request. I activated diagnostics mode and I noticed that you are getting an interesting error that appears to be referencing a stale mount link. It looks like the error is identical to the one described in the below thread:
I would try to focus on this solution, how was the USB drive formatted when you plugged it into the NUC, was it by any chance using NTFS? If it was using NTFS, can you try to format it as exFAT and see if it then shows up as a proper storage location in Roon? Thanks!